GOD IS ONE!
SESSION ONE
COMMON GROUND
Text: First Timothy 3:16
INTRODUCTION
This study deals with the common and major teachings of the Godhead.
There are three major beliefs as to the Godhead.
1.) Arianism–This teaching is found today in the Unitarian churches. (A variation is taught in the Jehovah Witnesses). The teaching is that Jesus was a good man but only divine in the sense that what he said was inspired of God. So the teaching is that there is one God but that Jesus is not God.
2). Trinitarianism–taught in one form or another in virtually all of the major denominations.
The Trinity
The doctrine of the Trinity is not a truth of natural theology, but of so-called extra-biblical revelation. Reason may show us the unity of God, but the doctrine of the Trinity comes from extra-biblical revelation. Though the term Trinity does not occur in the Bible, it had early usage in the church. Its Greek form, Trias, seems to have been first used by Theophilus of Antioch who died in A.D. 181. And this Latin form, trinitas, by Tertullian who died circa A.D. 220. In Christian theology, the term Trinity means that there are three eternal distinctions in the one divine essence, known respectively as Father, Son, and Holy Spirit. These three distinctions are three persons, and one may speak of the tri-personality of God. The Athanasian Creed expresses the Trinitarian belief thus, “we worship one God in the Trinity, and the Trinity in unity; we distinguish among the persons, but we do not divide the substance.”  It goes on to say, “the entire three persons are coeternal, coequal with one another, so that we worship complete unity in Trinity and Trinity in unity.”
The doctrine of the Trinity must be distinguished from both tritheism and Sabellianism.  Tritheism denies the unity of the essence of God and holds to three distinct gods. The only unity that it recognizes is the unity of purpose and endeavor. God is a unity of essence as well as of purpose and endeavor. The three persons are consubstantial. Sabellionism held to a Trinity of revelation, but not of nature. It taught that God, as father, is the creator and lawgiver; as son, is the same God incarnate who fulfills the office of Redeemer; and as Holy Spirit, is the same God in the work of regeneration and sanctification. In other words Sabellionism taught a modal Trinity as distinguished from an ontological Trinity. Modalism speaks of a threefold nature of God, in the same sense in which a man may be an artist, teacher, and a friend, or as one may be a father, a son, and a brother. But this is in reality a denial of the doctrine of the Trinity, for these are not three distinctions in the essence, but three qualities or relationships in one and the same person.
To be sure, the doctrine of the Trinity is a great mystery. It may appear to some as an intellectual puzzle or a contradiction. The Christian doctrine of the Trinity, mysterious as it may seem, is said to be an outgrowth of not speculation but of so called revelation. 
(Seems to be a way to explain the multifaceted God) (Have an article that I can give you.
3. The third major teaching on the Godhead is oneness–taught in many Pentecostal movements. It seems to have been established as a teaching almost immediately after the latter rain outpouring of the Holy Ghost at the turn-of-the-century. Oneness teaches that there is only one God who played three roles. This God was father in creation, son in redemption and Holy Spirit in emanation. The spirit of God emanates from the very being of God to our souls.
Note: There is a group that is referred to as Jesus only this is taught by one movement and is not oneness. Jesus only teaching is that God did manifest himself as father in creation, son in  redemption and Holy Ghost in emanation, but when God was in Christ, all of God was in Christ which is a heresy because it denies the omnipresence of God. Also, when we get to heaven the visible form of Jesus that will be there that we will see will be all there is of God. But the glorified body of Jesus will be the visible image of the invisible God even in heaven. God cannot be confined to one place and still be omnipresent. He relates to us through a visible body.
The two main teachings are oneness and trinitarianism. The others are heresies and unscriptural.