The first scripture I used in defense of the Trinity is a first for many, John 1:1 " and the word was God." Eph 4:4. Yes there is only one true God and many false Gods. The Bible also shows that God himself viewed some of the Israel kings and the judges of Israel as gods ( Exodus 21:8; Psalms 8:5; 82:1,6; 138:1) . He and Jesus viewed people as gods. (John 10:34,35) True we would not worship those gods, but they are gods in that they have supremacy and/or power, (the definition of God ). To say "Jesus is God or god", (there was no capitalization in "Greek", the original language) and therefore is the true God", is no more a fact than it is to say these others just mentioned also must be the true God.
A sample checking or the original language shows clearly "the word was with (ho theos) the God and the word was ( theos) god. Not `ho theos' or The God. The next verse again mentions `ho theos'. The Greek does not say "the word was (ho theos) the God", simply because Jesus is not `the God', as Trinitarians try to read into this scripture. See your average Greek commentary for more details. Same with Isaiah 9:6, "Jesus is called Mighty God (Hebrew `El Gibbohr') but not the Almighty God (not `El Shaddai').
Another scripture I used was John 10:30, where Jesus said "I and the Father are one." Also 1John 5:17 (overlooking the fact that this scripture is not any of the older original manuscripts) "these three are one". Jesus prayed in John 17:11, "that they may be one JUST AS WE ARE". Does that prayer mean we are God too? My wife Tammy and I are one (Matt 19:5) but Tammy is not me. The one is unity, purpose not literal . (John 17: 21)
Some say that "Jesus was condemned for blasphemy, so they thought he was God." WRONG! See John 19: 7& 10:36. Besides the Pharisees and Scribes, did not know what they were talking about anyway.
In Exodus 3:14 we read "Tell them I AM sent me to you" (Hebrew `Ehyeh') (In the Greek Septuagint, the contemporary translation of Jesus' day `ho on'). Jesus said in John 8: 38, "Before Abraham was I AM" (Greek `Ego eimi'). Unfortunately English translators use the same words in English for different words in the original languages. Jesus did not say he was "Ehyeh" or "ho-on" but said he was "Ego eimi"- something different, to say the least. By reading the context we find Jesus was merely saying he was around before Abraham was born. Being God's first creation, this would be a fact!
There are other scriptures used to try and support the Trinity, perhaps in time I'll list more.
One more thought of interest, since the Bible is very, unarguably clear when it says "God is Love" 1 John 4:28, or He is the God of Abraham, Isaac and Jacob. Ex. 6:3 or that "his invisible qualities are CLEARLY SEEN", Romans 1:20. God tells us so we have no doubt, that He parted the Red Sea, Ex.14, or that He gave us the Ten Commandments, Ex. 20, and so on. They are also clear that God wants us to get to know Him intimately. Acts 15:17; 17:27; Psalms 113:6; Jer. 23:23. So why would He not make it so clear, and tell us right out that He is `The Great Triune', 'The Holy Trinity' or varying expressions of this sort, especially when we consider how clear the bible is on small `insignificant' facts like Esau was hairy (Gen 25:25), Ehud was left-handed (Judges 3:15), Eli was fat (1 Sam. 4:18), King Saul was tall (1 Sam. 9:12), Elisha was bald (2 kings 2:23) ect.
Not even in the `gossip circles' (I use that term loosely) of bible days, did any one talk about Jesus being God or part of the Trinity. Matt 16:13,14; John 7:12, 40-44. They did, however, know he was the Christ, the SON of God! John 1:49; 6:14,68; 7:25-33 10:20-24; 11:27; 16:30; 19:7; 20:31; to list a very few.