The Trinity is under more and more of an attack by heretical groups. Moromon’s teach polytheism, that is, the Father, the Son and the Holy Spirit are three separate Gods. Not only are they three separate Gods but if you are a good enough Mormon you can become as God as well and own your own planet.
Jehovah’s Witnesses believe that there is one almighty God and that Jesus is a lesser God.
Oneness Theology teaches Modalism, that is, the Father the Son and the Holy Spirit are one person and manifests themselves into different modes, just like actors would in a play.
None of these views represents the Biblical presentation of who God is.
We must have a working definition of the Trinity which I will lay out for any misunderstanding one might have of the Trinity: There is only one God, This one God reveals himself as three co-eternal, co-equal persons, Father, Son and Holy Spirit. It would indeed be a contradiction if I said that one person is three persons or that one being is three beings, but rather one being is three persons. This makes all the difference. We as humans have being, human being, as well have having personal attributes as well.
Yet in our physical existence there is nothing that has three persons in one being. Many try to use the example of the egg yolk, the shell and the egg white, however, this better explain the Mormon gods than the Christian God. Others use the example of water in its three different forms liquid, ice and steam. This also teaches heresy and better explains the oneness God who manifests himself in three different forms.
We see that in Deuteronomy 6:4 that there is one God “Hear, O Israel: The Lord our God, the Lord is one.” And also Isaiah 45:5-6 makes it very clear “I am the Lord, and there is no other, besides me there is no God; I equip you, though you do not know me, that people may know, from the rising of the sun and from the west, that there is none besides me; I am the Lord, and there is no other.” Therefore Polytheism (multiple gods) is a foreign idea in scripture and also forbidden (Exodus 20:3).
We see the Deity of three persons. Psalm 89:26 communicates the Father as God “He shall cry to me, ‘You are my Father, my God, and the Rock of my salvation.” Colossians 2:9 communicates the Son as God “For in him [Jesus] the whole fullness of deity dwells bodily,” and Acts 5:3-4 Communicates The Holy Spirit as God “Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you have contrived this deed in your heart? You have not lied to men but to God.”
We also see a Distinction between persons when Jesus prays he is not simply praying to himself nor is his humanity praying to his divinity, he is praying to another person, the Father. See John 17 or in the profound statement Jesus makes in Matthew 27:46 quoting from Psalm 22 “Eli, Eli, lema sabachthani? That is, “My God, my God, why have you forsaken me?” Also in Matthew 11:27 you see a clear distinction of the Father and the Son “All things have been handed over to me by my Father, and no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him.” On top of all this we see that they work differently in John 5:22 The Father doesn’t Judge but the Son does. In John 6:37-38 we see the Father giving the son people to come to him and Jesus came to do the Father will. So the Distinctions between persons rules out modalism/oneness theology, God doesn’t dress up in different costumes to perform different acts like in a play.
Summary: The Bible clearly states there is ONLY ONE God, yet throughout the Bible it calls the Father, God, the Son, God and the Holy Spirit, God. Since we hold that the Bible is God breathed and without error, we do not see a contradiction, but rather a complex explanation of the being of God. We do not pretend to know everything about Gods nature, yet the Trinity is clearly spelled out in the Bible where we can know that He is one God yet three persons. We should not expect an infinite being to be easily comprehended or compared to something that comes from the rear end of a chicken.
Also see Plurality of God in the OT and NT by Matt Slick