The trinity is defined as one God manifested in three persons. This is only one of many things we can’t understand completely about God. If we did understand everything about God, God wouldn’t be God, would he?
Just as the word ‘Bible’ isn’t found in the Bible, neither is the word ‘trinity.’ But the concept is clearly taught! Lets look at some scriptures that clearly state there is only one God, and three separate and distinct persons are that one and only God.

There is only one God:
Isaiah 43:10 You are my witnesses, says the LORD, and my servant whom I have chosen, so that you may know and believe me and understand that I am he. Before me no god was formed, nor shall there be any after me. (NRSV) See also Deuteronomy 6:4;32:39 and Isaiah 44:8
Isaiah 45:22 For I am God, and there is no other. (NRSV)
1 Corinthians 8:4 we know that “no idol in the world really exists,” and that “there is no God but one.” (NRSV)

The Father is God:
2 Peter 1:17 For he received honor and glory from God the Father when that voice was conveyed to him by the Majestic Glory, saying, “This is my Son, my Beloved, with whom I am well pleased.” (NRSV) See also John 6:27 and Ephesians 4:6

The Son is God:
John 1:1-14 In the beginning was the Word, and the Word was with God, and the Word was God. ² He was in the beginning with God. ³ All things came into being through him, and without him not one thing came into being. (NRSV) Verse 14 tells us ‘the word’ is Jesus.
Colossians 1:15-19 also tells us Jesus created everything. Hebrews 1:6 tells us that the angels worship Jesus as God. In Phil 2:5-6 tells us that Jesus temporarily gave up his equality with the father to become human. See also John 5:17-18;10:33

The Holy Spirit is God:
Acts 5:3-4 Peter asked, “why has Satan filled your heart to lie to the Holy Spirit and to keep back part of the proceeds of the land? ⁴ While it remained unsold, did it not remain your own? And after it was sold, were not the proceeds at your disposal? How is it that you have contrived this deed in your heart? You did not lie to us but to God!” (NRSV)
Luke 1:35 The angel said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be holy; he will be called Son of God.” (NRSV)
The Holy Spirit was active in creation;
Genesis 1:2 ...And the Spirit of God moved upon the face of the waters. (KJV)
John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. (KJV)
The Holy Spirit prophesies;
1 Timothy 4:1 Now the Spirit expressly says that in later times some will renounce the faith by paying attention to deceitful spirits and teachings of demons, (NRSV)
Romans 8:11 the Holy Spirit resurrects us.
Matthew 28:19 we are to baptize in the name of the Father , Son, and Holy Spirit.
Acts 16:16 Paul was forbidden to go to Asia by the Holy Spirit.
Acts 13:4 Paul was sent to Seleucia by the Holy Spirit.
Acts 13:2 the Holy Spirit commanded Paul and Barnabas to be missionaries.
Acts 21:11 the Holy Spirit speaks by Agabas
Acts 28:25 the Holy Spirit speaks truth.

They exist separately at the same time:
Matthew 3:16-17 And when Jesus had been baptized, just as he came up from the water, suddenly the heavens were opened to him and he saw the Spirit of God descending like a dove and alighting on him. ¹⁷ And a voice from heaven said, “This is my Son, the Beloved, with whom I am well pleased.” (NRSV)

We are warned about another Jesus, Holy Spirit, and gospel:
Galatians 1:6-8 I am astonished that you are so quickly deserting the one who called you in the grace of Christ and are turning to a different gospel -- ⁷ not that there is another gospel, but there are some who are confusing you and want to pervert the gospel of Christ. 8 But even if we or an angel from heaven should proclaim to you a gospel contrary to what we proclaimed to you, let that one be accursed! (NRSV)
2 Corinthians 11:3,4,13-14 But I am afraid that as the serpent deceived Eve by its cunning, your thoughts will be led astray from a sincere and pure devotion to Christ. ⁴ For if someone comes and proclaims another Jesus than the one we proclaimed, or if you receive a different spirit from the one you received, or a different gospel from the one you accepted, you submit to it readily enough. ¹³ For such boasters are false apostles, deceitful workers, disguising themselves as apostles of Christ. ¹⁴ And no wonder! Even Satan disguises himself as an angel of light. (NRSV)
It’s one thing to be Buddhist, Hindu, Muslim, or any other religion, but it’s quite another to lie and say your Christian when your not. Cults mix beliefs from different religions together, and are considered deceivers by those religions. Christians are commanded to, as Jude 3 says, contend for the faith that was once for all entrusted to the saints. If you say you believe in the trinity but add to it by saying we are evolving into gods or already are gods then you are considered a Polytheist. Polytheists and antitrinitarians are not considered Christians, but Cults.

Here is a partial list of antitrinitarian groups:
Garner Ted Armstrong, Bahai, Black Muslims, Branch Dividians, William Branham, Children of God “The Family”, Christian Science, Christadelphians, Church of the Living Walk “John Robert Stevens”, Church Universal and Triumphant “Elizabeth Clare Prophet”, Divine Light Mission, Divine science, ECK-ANKAR, EST, Hare Krishna’s, Jehovah’s Witnesses - The Watch Tower Bible and Tract Society, The Local Church “Witness Lee”, Charles Manson, Roy Masters, Metaphysics, New Age, Occult Philosophy, People’s Temple “Jim Jones”, Rajneesh, Religious Science, Reverend Ike, Rosicrucians, Sai Baba, Satanism, Scientology “Dianetics”, Self Realization Fellowship, Shakers, Silva Mind Control, Spiritism, Transcendental Meditation, Unification Church “Sun Myung Moon”, Unitarianism, Unity, Universal light, UPC “Oneness Pentecostals”, Uranta, The Way International, Worldwide Church of God “Herbert W. Armstrong”

Here is a partial list of Polytheistic groups:
Kenneth Copland, Kenneth Hagan “Rhema”, Mormonism “RLDS-LDS-Temple Lot-Strang-Brickerton”

All Biblical quotations taken from the NRSV unless otherwise noted.