This is the Nicene Creed as it was produced by the Council of Nicaea (or Nicea or even Nice). There were additions made at later councils, especially to the line about the Holy Spirit.
We believe in one God, the Father Almighty, Maker of all things visible and invisible; and in one Lord Jesus Christ, the Son of God, the only-begotten of the Father, that is, of the substance of the Father; God of God and Light of light; true God of true God; begotten, not made, consubstantial with the Father, by whom all things were made, both which are in heaven and on earth: who for the sake of us men, and on account of our salvation, descended, became incarnate, and was made man; suffered, arose again the third day, and ascended into the heavens, and will come again to judge the living and the dead; also in the Holy Spirit. But the holy, catholic [i.e., "universal," not Roman Catholic], and apostolic church anathematizes those who say, "There was a time when he was not," and "He was not before he was begotten," and "He was made from that which did not exist," and those who assert that he is of other substance or essence than the Father, that he was created, or is susceptible of change.
I will explain and give the history line by line.
We believe in one God, the Father Almighty, Maker of all things visible and invisible.
The Nicene Creed was not invented from nothing in A.D. 325. It was based on the "rule of faith" from the church in Caesarea. Before the Council of Nicaea, every church had a statement of faith, taught at baptism.
At least some churches, if not all, would immerse a person three times, once in the name of the Father, once in the name of the Son, and once in the name of the Holy Spirit. The names were not simply pronounced over the person, but a question was asked. For example, using the Nicene Creed, which was simply a "rule of faith" agreed on by all the churches of the Roman Empire, the person baptizing would ask, "Do you believe in one God, the Father Almighty, Maker of all things visible and invisible?" The one being baptized would respond with "yes." This would be repeated for the Son and Spirit, and yes, the person being baptized would get a very long question in regard to the Son, as you can see in the second part of the creed, then be immersed a second time. Finally, they would be asked if they believe in the Holy Spirit and immersed a third time. You will notice below that the original Nicene Creed had no description for the Holy Spirit, just an acknowledgment that the church believed in the Holy Spirit. We will discuss that further below.
The "one God" is described as "the Father Almighty." We are used to describing the one God as Father, Son, and Holy Spirit, but this is neither the Council of Nicaea's way of describing the one God, nor Scripture's. This terminology is taken straight from 1 Corinthians 8:6, where Paul writes:
But for us there is but one God, from whom are all things ... and one Lord, Jesus Christ, through whom are all things. (NASB1995)
That can be shocking for Christians not used to the idea that the Father is the one God, both in Scripture, in the writings of the early church fathers, and in the Nicene and Apostles' creeds. It would be good for us to correct our terminology to biblical terminology, but it takes little change in theology. The wording that the Father is the one God simply implies that the Father is the source of the Trinity. The Son is the only-begotten Son of the one God, sharing the same divinity or "essence" of the Father (see below), and the Holy Spirit proceeds from the Father.
This does explain one crucial fact of history. In the latter part of the first millennium, believing that the pope had the authority to establish doctrine, the Roman Catholic Church added one Latin word to the Nicene Creed. The first Council of Constantinople, A.D. 381, had added "the Lord, and Giver of Life, Who proceeds from the Father, Who with the Father and the Son together is worshipped and glorified" to the creed. This was later approved by most of the bishops of the Roman Empire at the Council of Chalcedon in 451. The Roman church later added filioque, meaning "and the Son" to the creed, so that it read, "We believe in the Holy Spirit who "proceeds from the Father and the Son."
This takes away from the idea that the Father is the source of the Trinity, a belief which the corrupt and politicized Roman Catholic hierarchy had forgotten, and led to the mutual excommunication between the exalted bishops of Rome and Constantinople and the split that produced the names "Roman Catholic and Eastern Orthodox."
And [we believe] in one Lord Jesus Christ, the Son of God, the only-begotten of the Father. That is, of the substance of the Father; God of God and Light of light; true God of true God; begotten, not made, consubstantial with the Father. By [him] all things were made, both which are in heaven and on earth: who for the sake of us men, and on account of our salvation, descended, became incarnate, and was made man; suffered, arose again the third day, and ascended into the heavens, and will come again to judge the living and the dead.
Let's parse these statements one by one.
"... one Lord Jesus Christ, the Son of God"
"Lord" and "Christ" are extremely important terms. By the fourth century the bishops at Nicaea may have been as desensitized to the words as we are today. In the first sermon at Pentecost, Peter concluded with "Therefore, let all the house of Israel know assuredly that God has made this Jesus, the one you crucified, both Lord and Christ" (Acts 2:36).
Peter was addressing a Pentecost crowd of Jews from both Jerusalem and outside Jerusalem. Those far from Jerusalem were often Greek-speakers, called "Hellenists" in Acts 6:1. They would have been aware that "Caesar is Lord" was a declaration of the divinity held by the Roman emperor. Peter's claim that Jesus is "Lord" exalted him above Caesar.
The claim that he is "Christ" exalts him above all rulers and nations everywhere. Earlier in his discipleship to Jesus, Peter had described Jesus as "Christ and Son of God" (Matt. 16:16). There is only one place in the Bible that "Christ" and "Son of God" are mentioned together: Psalm 2. Both "Christ" and "Messiah" literally mean "Anointed One." Most translations use "anointed one" in Psalm 2:2, but the Hebrew word is "Meschiach," translated "Messiah" in other Old Testament passages. In other words, "the LORD and his Anointed" in Psalm 2:2 is God and the Messiah/Christ. So when Peter referred to Jesus as "Christ and Son of God" in Matthew 16 or as "Lord and Christ" in Acts 2:36, he was referring to the Anointed King of Psalm 2 who would possess the nations and the uttermost parts of the earth (Ps. 2:8).
Of course, we know from the apostles that the "rod of iron" for all nations is reserved for the second coming. Until then, though, Jesus is still Lord and Messiah. He is still higher than all the kings of the earth and, as Napoleon Bonaparte famously pointed out, Jesus' kingdom is greater than all others because he has conquered and subdued by love alone. (The full text of Napoleon's description of Jesus and his kingdom, found at the link I just gave, is well worth reading.)
"... the only-begotten of the Father"
In the west, we have forgotten that Jesus was begotten by the Father before the beginning. When we read only-begotten, we think of Bethlehem. The bishops of Nicaea thought of the emptiness before creation, when God was alone and nothing existed outside of him. As Athanasius, the great champion of the Nicene Creed, put it:
We believe in one Unbegotten God, Father Almighty ... and in one Only-begotten Word, Wisdom, Son, begotten of the Father without beginning and eternally. ("Statement of Faith" 1).
The doctrine of the Trinity, as taught at Nicea and by the apostles and the bishops and elders who preserved the teaching of the apostles, hinged on the teaching that Jesus was begotten before the beginning. (My book, Decoding Nicea, uses quotes from the beginning of church history to establish that the Council of Nicaea affirmed the Trinity as it had been taught from the beginning. Just the quotes on my Trinity quotes page will establish that fact.)
Not only has this teaching, that the Son was begotten of God before the beginning, been forgotten in the West, but it also seems to be purposely attacked in some modern Bible translations. Some Greek scholars now argue that monogenes does not mean "only-begotten," but just "unique." If this is true, then Eusebius, who wrote his Ecclesiastical History in Greek in 323, did not actually understand Greek because he wrote:
The true Son of God, forasmuch as he is begotten of the Father, is properly denominated the monogenes and beloved of the Father. For this reason also, he himself is God; for what can the offspring of God be, but the perfect resemblance of him who begot him? (Against Marcellus, as cited by The Ecclesiastical History of Socrates Scholasticus, bk. II, ch. 21)
This citation not only defends monogenes as meaning "only-begotten," but it explains how critical the doctrine is to the Trinity. Jesus is God because he is the offspring of God. Many early Christians explain that the reason that the Father can be God and Jesus also called God, yet there be only one God, is because there is only one divinity. The Son was begotten of the Father like a stream is begotten from a spring, or like a sunbeam is begotten of the sun. The spring and the stream are two things with two names, but the water that flows from one and into the other is one and undivided. So the "substance" of God is one and undivided between the Father and his only-begotten Son.
You can find the illustrations I just mentioned among the quotes on my Trinity quotes page, which I linked above as well.
You will find a defense of monogenes as only-begotten by a professor of Biblical Studies at "Deep in the Weeds on Monogenes and Eternal Generation." I want to point out two things, though. The mistranslation of monogenes into "unique" or "only" can produce a nonsensical translation of John 1:18, where the English Standard Version has:
No one has ever seen God; the only God, who is at the Father's side, he has made him known.
The only God is at the Father's side? Did no one on the translation committee notice that this translation denies the foundational truth of Christianity, that there is one God, the Father? (1 Cor. 8:6). There is a textual issue in John 1:18. Some manuscripts have "the monogenes Son." If the ESV had translated John 1:18 as, "... the only Son, who is at the Father's side," there would be no nonsense and no gross heresy. Better, though, is translating monogenes correctly and using whichever manuscript you prefer because both "... the only-begotten God, who is at the Father's side" and "... the only-begotten Son, who is at the Father's side" are completely orthodox.
Secondly, those who would deny the begetting of the Son before the beginning cannot eliminate all references to it by mistranslating monogenes. Paul uses prototokos, "firstborn" over all creation, to describe Jesus in Colossians 1:15.
"... that is, of the substance of the Father; God of God and Light of Light; true God of true God; begotten, not made, consubstantial with the Father"
This super specific description of the begetting of the Son was prompted by the heresy of Arius. Arius agreed that the Son was begotten by the Father before the beginning, but he claimed this was the equivalent of being created. In his eyes, the Son was created from nothing, just like everything else was. This was not the teaching of the church. Instead, the church taught that the Son was truly "begotten" in the sense that he came out of the Father's bosom. He was indeed like a stream from a spring, originally inside the Father, then going forth from him. "Begetting" or being born is the best we humans can do with an idea that is beyond our understanding, but "begetting" is the word God chose to give to us in the Scriptures.
Thus, the council used "the substance of the Father" and "God from God" and "Light from light" to describe him.
I need to point out that the Nicene Creed was written in Greek, and "consubstantial" was the controversial word homoousios. The bishops were not really open to the word because a famous heretic named Paul (of Samosata) had used it justify his "monarchian" theology (i.e., only the Father was God). On the other hand, Athenagoras the apologist, around the same time, wrote, "We acknowledge a God, and a Son, his Logos, and a Holy Spirit, united in essence, the Father, the Son, the Spirit" (A Plea for the Christians ch. 24).
I have not been able to find the Greek of the sentence, but if he did not use homoousios itself, which means "one in essence," he used something similar to indicate "united in essence" . There are two points here. The council added a word that would represent the teaching of the Trinity, but which was not used in any church's rule of faith. There was danger the word could be considered a novelty, and since the duty of the bishops was to preserve what the apostle taught, not add to it, novelty was not acceptable. The second point is that the term did line up with the teaching that early Christians testified had come from the apostles.
I should also tell you that Eusebius, who had finished his Ecclesiastical History (or just Church History) two years earlier and who was at the council, says that it was Emperor Constantine who pushed for the inclusion of the word homoousios.
"... by [him] all things were made, both which are in heaven and on earth"
This confirmed that God created all things through his Word (John 1:1-3), Jesus Christ.
"... who for the sake of us men, and on account of our salvation, descended, became incarnate, and was made man; suffered, arose again the third day, and ascended into the heavens, and will come again to judge the living and the dead."
This simply confirms the things Scripture says about his mission, his death, his resurrection, the atonement, the second coming, and the final judgment. Since the idea was that this creed would be learned by all those who would be baptized in the future, it ensures that a new believer believed the right Gospel and did not deny the physical birth, life, death, resurrection, and ascension of Jesus. Gnostic heretics did deny these things.
[We] also [believe] in the Holy Spirit.
It was interesting to me that the Nicene Creed, as it was released by the Council of Nicaea in 325, said no more than this about the Holy Spirit. In my book, Decoding Nicea, I point this out and express my gratitude at not having to deal with the things said in Scripture and by the fathers who wrote before Nicaea about the Holy Spirit. While the divine Trinity is confirmed in both Scripture and the pre-Nicene fathers, the explanations about the relationship between the Holy Spirit and the Father and Son are not near as clear nor as consistent as the things written about the Father and Son.
But the holy, catholic, and apostolic church anathematizes those who say, "There was a time when he was not," and "He was not before he was begotten," and "He was made from that which did not exist," and those who assert that he is of other substance or essence than the Father, that he was created, or is susceptible of change.
Most often, these anathemas are not included at baptisms or when the Nicene Creed is recited. As said, the Council of Constantinople made some adjustments to the creed that were confirmed at the Council of Chalcedon, including a longer description of what believing in the Holy Spirit means. The updated creed is known as the "Nicene-Constantinopolitan Creed," which makes me chuckle at the length of the name, and which does not include the anathemas.
These anathemas were specifically directed at the Arian heresy. Under Emperor Theodosius I, Arian doctrine was specifically rejected and Arian churches were barred from the major cities of the empire.
One strange-but-true story is that Theodosius' predecessor, Valens, had supported Arianism. During his reign, he had intervened in a conflict between barbarian tribes. When the tribe he supported conquered the other, they agreed to convert to Valens' religion, Arian Christianity. Over the next eighty years or so, those same tribes would conquer both the western empire and the city of Rome itself. These Christian tribes deposed the emperor in Rome, but they considered the Roman bishop the representative of and the authority over Christians and churches in the West, so they did not depose him.
The Roman bishop, "the pope," brought them back to Nicene Christianity, but over the centuries, both the Roman Catholic Church and the Protestants who descended from them lost the Nicene definition, forgetting (or rejecting the idea?) that the Father is called the one God and is the source of the Trinity. There are notable exceptions, such as some Anabaptists and John Wesley's friend, Ian Fleming, who wrote a book on the council. (I can't seem to remember the name or find it online. It was a lot like Decoding Nicea, my book, but mine is in more modern English, of course. If you know the title, please put it in the comments.)
I sure hope all this was helpful. Feel free to provide corrections to my history. I wrote a lot of this from memory, whereas when I was writing Decoding Nicea, I had a lot of notes available.
I compiled and explained a huge number of quotes on the Trinity in Decoding Nicea. I put those two chapters, a total of 68 pages online as a PDF (free).