Is Trinity Biblical?

I have a book, written by a very intelligent man named Walter Martin, called "The Kingdom of the Cults." Very early on in this book, there is a list of things to help identify a cult. The very first thing on the list is that what the organization teaches must be biblical. If it's in the Bible, and the organization teaches against it, or if it's not in the Bible, but the organization does teach it, then that organization is a cult. That makes very good sense to me. That seems like a very solid proof. The second item on the list is that the organization must teach Trinity. That puzzles me. First of all, if Trinity is biblical, then that idea is covered under item 1, so item 2 is redundant; on the other hand, if Trinity isn't biblical, then item 2 contradicts item 1. Now, I've already mentioned that the late Walter Martin was a very intelligent man, but I have to wonder, why is this even listed?

I think that it's because the evidence for Trinity in the scriptures is somewhat tenuous. The Thompson Chain-Reference Bible lists 8 verses under the topic 'Trinity.' The word Trinity itself doesn't appear in scripture (of course, neither do a lot of other words that we use regularly in church). Really, it seems to me that there are eleven verses that could be used to support the Trinity doctrine: Matthew 28:19, John 14:26, John 15:26, Acts 7:55, Acts 10:38, Romans 15:16, 2 Corinthians 13:14, Galatians 4:6, Ephesians 1:17, 1 Peter 1:2, and 1 John 5:7. For something that so many people consider to be so important, that seems to be pretty thin. If God wanted us to believe in Trinity, wouldn't that be a bold statement somewhere in the Gospels ("Behold, I am the LORD, and my Name is Three!")?

I can come up with a lot more than eleven scriptures that could be used to argue the Oneness of God, as opposed to the Triune formula. Realistically, I could use some of the eleven scriptures listed above to argue against Trinity as easily as to argue in its favor. Some of them, taken together, can get pretty confusing: For example, in John 14:26, Jesus says that the Father will send the Comforter in Jesus' name, but in John 15:26, Jesus says that He himself will send the Comforter from the Father. So, did Jesus send us the Comforter, or did the Father send us the Comforter? And if that weren't confusing enough, in John 14:18, Jesus indicates that He (Jesus) is the Comforter, but 8 verses later, He tells us that the Comforter is the Holy Ghost.

Furthermore, what does Trinity really mean? To some people, Trinity is simply shorthand for the three major manifestations of God, to others, it means that there are three separate Gods; still others believe that the Father and the Holy Ghost are Gods, but the Son is simply the Son of God, and not really a God in his own right (but we can't ignore Him, because He died for our sins). The use of the term Trinity is very widespread, but there doesn't seem to be any real consensus as to what Trinity is. How can this be a defining point between a legitimate Christian church and a cult if we can't even decide what it is?