This is the first post in the beginning of the second set of articles in my ongoing series, theology in bullet points. The first set had to do with God and Creation. This second set has to do with Christ and Salvation.
1. In theology, the study of Christ is called Christology. It primarily concerns the "person of Christ," although it is sometimes difficult to discuss the "person" of Christ without also discussing the "work" of Christ. The early Christian debates of the 300s and 400s dealt extensively with questions about the person of Christ and how he related to God the Father. The creeds that came from these debates set down clear parameters for what Christians should believe.
These creeds, short though they are, reflect centuries of agonizing discussion in the early church. In the New Testament, we have some hints of things the early Christians said or "confessed" about Christ. We sometimes call these sorts of statements, credos, after the Latin for "I believe." One of the earliest was probably the simple affirmation, "Jesus is Lord" (e.g., Rom. 10:9; Phil. 2:11; 1 Cor. 12:3).
Some form of what would become the "Apostle's Creed" probably existed by the year AD200. Its statements about Jesus are in somewhat of a story or narrative form:
"I believe... in Jesus Christ, his only Son, our Lord, who was conceived by the Holy Spirit, born of the Virgin Mary, suffered under Pontius Pilate, was crucified, dead, and buried. He descended to the dead. On the third day, he rose again from the dead. He ascended into heaven and sits on the right hand of God the Father, Almighty. From there he will come to judge the living and the dead."
This is a good summary of the story of Jesus as we find it in the Gospels.
2. However, this story line did not answer the kinds of questions that arose after the New Testament period. Is Jesus the most exalted of God's creations, or is he God himself in some sense? If he is truly God, then how does his divinity fit with his humanity? Indeed, the New Testament itself is not entirely clear on these sorts of questions, arguably because the New Testament authors just weren't asking them.
There were faith-filled individuals in the early centuries of Christianity who argued both sides of these debates from Scripture, just as today different groups argue contradictory things from the same Scripture. This is a significant point. Some Christians who would be "heretics" now (individuals who disagree with core Christian dogmas) were not yet heretics when they argued for certain positions back then. That is because "orthodoxy," or right belief, was not yet established at that time.
For example, a faith-filled believer named Sabellius argued in the early 200s that Jesus was actually the same person as God the Father and the Holy Spirit. These were just three different faces or three different manifestations of one person changing roles and appearances.  In the Old Testament he appeared as God the Father. Then he came to earth as Jesus. Then he came back now as the Holy Spirit. To Sabellius, these were not three persons--as most Christians have always believed--but only one changing hats.
From the perspective of orthodoxy--the common consensus of Christians since the earliest centuries of Christianity--there are a couple positive aspects to what Sabellius suggested. First, he maintains what was essential even to Israelite faith in the Old Testament--there is only one God (e.g., Deut. 6:4; 1 Cor. 8:6). This position is the bottom layer of biblical confessions about God and was thus always the bedrock of this discussion in early Christianity. There is only one God, not three. 
Sabellius also believed that Jesus was fully and 100% God, which is the position the Church would eventually conclude in the 300s. No doubt Sabellius looked to verses like John 10:20 as the basis for this belief. There Jesus says, "I and the Father are one." There were those who interpreted this verse differently. After all, two people can be in complete agreement and say things of this sort. Two kings might be unified on a policy and say, "We are one on this question."
Indeed, there appear to have been some forms of Jewish Christianity in the 100s who believed that Jesus was the Messiah but not that he was God in any sense like God the Father was God. There was a group known as the Ebionites, for example, and another called the Nasarenes, which was still in existence in the 400s.  And here we also need to remember that the Roman emperors were known as divine as well. The memoir of the first emperor, Augustus, is titled, "The Story of the Divine Augustus."
So Sabellius was orthodox in his belief in one God, and he sided with what would become the orthodox belief that Jesus was fully God. Where he would later be considered unorthodox is in his sense that Jesus and God the Father were not two distinct persons but the same person.
3. Another position that would be considered wrong by the end of the 300s is that of a church leader named Arius.  Arius correctly taught that there was only one God, but he did not include Jesus within him. On the one hand, Arius did believe that Jesus had existed before he came to earth. That is to say, Arius believed in the pre-existence of Jesus.
There are a number of passages in the New Testament that have led most scholars to conclude that Paul and certainly John believed that Jesus had existed before he came to earth.  John of course clearly states this position in statements like John 17:5: "Father, glorify me in your presence with the glory I had with you before the world began."  In Paul's writings, Philippians 2:6-8 are usually taken to refer to Jesus coming to earth from heaven: "Though he was in the form of God... he emptied himself... being born in human likeness (NRSV)."
Arius believed all this. Indeed, Arius affirmed strongly on the basis of Scripture that Jesus was the "firstborn of all creation" (Col. 1:15, NASB). He believed that Jesus was Lord over all creation, much higher than the angels. He believed that he could affirm everything said about Jesus in Scripture.
But Arius placed Jesus on the creation side of the dividing line. Jesus was the most exalted of all God's creation but still a creation. He taught that, "There was a point when the Son did not exist," a point before time. He might have said about Christ something like what the Jewish thinker Philo said about the Word of God--"I am neither uncreated like God nor created like you, but midway between the two extremes" (Heir 206).
The debate came to a head at the Council of Nicaea in AD325, the first universal council of Christianity. It was called by the Roman emperor Constantine, who had just finally declared Christianity a legal religion in AD313. It was important to him that Christians not fight among themselves but come to some point of agreement on this question.
The champion of the other side, the side that would eventually prevail by the end of the 300s, was Athanasius. The doctrine of the Trinity is the orthodox position of Christianity in large part because of his doggedness on the issue. No compromise position was accepted, such as the suggestion that Jesus was of similar substance to God the Father (homoiousios). Athanasius was insistent: Jesus was of the same substance as God the Father (homoousios).
Athanasius won at the Council, but it would take the better part of the 300s for his position to win in the churches of the empire. Nevertheless, in 381, the second "ecumenical" or worldwide council produced the Nicene Creed at Constantinople, which gives the position that Christians have generally held ever since:
"I believe in one Lord Jesus Christ, the only-begotten Son of God, born of the Father before all ages.
God from God, Light from Light, true God from true God, begotten, not made, of one substance with the Father..."
4. So we finally get to the claim of this article. Christians believe that Christ has been God since eternity past. He is "of one substance" (homoousios) with God the Father. He is not like God; he is God.
He is "begotten, not made." Arius argued that Jesus was made, part of the creation. Athanasius pointed to language in John of Jesus being "begotten." There is a difference, Athanasius argued. "Made" seems to imply that Jesus came into existence at some point. But Jesus was "eternally begotten." He has always been the Son who comes from the Father. They have always existed in this relationship.
A later creed from the 500s called the Athanasian Creed spells it out even more clearly:
"We worship one God in Trinity, and Trinity in Unity; neither confusing the persons nor dividing the substance. For there is one person of the Father, another of the Son, and another of the Holy Spirit. But the Godness of the Father, of the Son, and of the Holy Spirit is all one, the glory equal, the majesty coeternal. ..
"The Father is eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three eternals but one eternal. So similarly the Father is almighty, the Son almighty, and the Holy Spirit almighty. And yet they are not three almighties, but one almighty. So the Father is God, the Son is God, and the Holy Spirit is God; and yet they are not three Gods, but one God...
"The Father is made of none, neither created nor begotten. The Son is of the Father alone; not made nor created, but begotten. The Holy Spirit is of the Father and of the Son; neither made, nor created, nor begotten, but proceeding. So there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits. And in this Trinity none is before or after another; none is greater or less than another."
So common Christianity affirms that there are three distinct persons who constitute the one God. This is a mystery. It affirms everything that needs to be affirmed and leaves the rest unsaid, for any attempt to clarify more seems to end in a position that the early Christians rejected.
5. So Christians historically believe that Jesus Christ is a distinct person from God the Father but they both are the one God. They have existed eternally in the relationship of Father and Son.  According to historic Christianity, Jesus is not even subordinate to God the Father, as you can see in the final line above from the Athanasian Creed. They are "of one substance."
Christians believe that Christ has been true God, of one substance with God the Father, although a distinct person in the Trinity, from eternity past.
Next week: C2. Jesus is God's Word become flesh.
 In addition to the name "Sabellianism," this approach is also called "modalism" or "monarchical modalism," because God is just appearing in different "modes" in history, even though it is just the same person. In the church today, "Oneness Pentecostals," who do things in the name of "Jesus only" are modalists of this sort (e.g., the United Pentecostal Church). They do not believe in the Trinity.
 Belief in more than one god is called polytheism, and it has never been a serious option within Christianity, although it no doubt took place on a popular level after Christianity became the only legal religion of the Roman Empire in 380 under Theodosius I. Polytheism then went underground for some time among people who officially had to call themselves Christians.
 The Nasorenes (which I spell this way from Jerome, Epistle 19, so as not to confuse them with modern day Nazarenes, another spelling) even believed in the Virgin Birth.
 His thinking is thus called, "Arianism."
 There is surprisingly more debate on these issues among New Testament scholars than you might think. For example, it was nearly the consensus in the 1970s among New Testament scholars that Paul did not teach the pre-existence of Jesus. See James D. G. Dunn, Christology in the Making: A New Testament Inquiry into the Doctrine of the Incarnation, 2nd ed. (Grand Rapids: Eerdmans, 1996).
 John is unique among the Gospels in having statements like this one. The other three Gospels, usually called the "Synoptic" Gospels because they give such similar presentations of Jesus' earthly ministry, do not explicitly say anything about Jesus' pre-existence. We might easily miss the fact, for example, that while Matthew and Luke tell of the Virgin Birth, they say nothing explicitly about Jesus before his birth. For an argument that the Synoptics imply Jesus' pre-existence, see Simon Gathercole, The Pre-Existent Son: Recovering the Christologies of Matthew, Mark, and Luke (Grand Rapids: Eerdmans, 2006).
 Remembering that these genders are metaphorical. No genitalia is involved, meaning that God is not literally gendered.