Let’s remember the key traits of the Trinity. According to the Athanasian Creed,

The Father unlimited; the Son unlimited; and the Holy Ghost unlimited. …
So likewise the Father is Almighty; the Son Almighty; and the Holy Ghost Almighty. And yet they are not three Almighties; but one Almighty. So the Father is God; the Son is God; and the Holy Ghost is God. …
And in this Trinity none is before, or after another; none is greater, or less than another. …

About Jesus, it says:

Perfect God; and perfect Man …
Who although he is God and Man; yet he is not two, but one Christ.

Okay, okay, I get it. Three persons, all equal. None greater than another. Jesus is unlimited, almighty, and perfect.
But does the Bible agree? Remember that, unlike the clear definition of monotheism in the Koran, the doctrine of the Trinity is not clear. It took almost four centuries to congeal.
Consider some Bible verses that challenge the Trinitarian concept as defined in the creed above. First, verses that portray Jesus as an ordinary person who didn’t know everything, who wasn’t 100% with the program, and who spoke to God as you or I would.

When Jesus heard this, he was amazed (Matt. 8:10)
[Jesus] turned around in the crowd and asked, “Who touched my clothes?” (Mark 5:30)
[Jesus prayed,] “My Father, if it is possible, may this cup be taken from me. Yet not as I will, but as you will.” (Matt. 26:39)
Jesus cried out in a loud voice, “My God, my God, why have you forsaken me?” (Mark 15:34)
Jesus often withdrew to lonely places and prayed. (Luke 5:16)
You are looking for a way to kill me, a man who has told you the truth that I heard from God. (John 8:40)
[Jesus said,] “I am ascending to my Father and your Father, to my God and your God.” (John 20:17)

Verses that state that only God has certain traits or abilities.

No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father. (Matt. 24:36)
[Jesus said,] “The most important [commandment is:] The Lord our God, the Lord is one.” (Mark 12:29)
God, the blessed and only Ruler, the King of kings and Lord of lords, who alone is immortal and who lives in unapproachable light (1 Tim. 6:15–16)

Verses that portray Jesus as inferior to God.

“Why do you ask me about what is good?” Jesus replied. “There is only One who is good.” (Matt. 19:17)
[Jesus said,] “the Father is greater than I” (John 14:28)
The head of every man is Christ, and the head of the woman is man, and the head of Christ is God. (1 Cor. 11:3)
The Son himself will be made subject to him who put everything under him, so that God may be all in all (1 Cor. 15:28)
You have been raised with Christ, set your hearts on things above, where Christ is, seated at the right hand of God. (Col. 3:1, see also 1 Peter 3:22)
There is one God and one mediator between God and mankind, the man Christ Jesus (1 Tim. 2:5)

Ever-nimble Christian apologists have had 2000 years to find responses to many of these. Perhaps they’ll argue that we’re seeing the limited Man side of Jesus here, not the God side. Or that other verses can be brought in to bolster the Christian position. Nevertheless, the simplest explanation is that the Bible is a collection of books from authors (many unknown) who had similar but not identical religious beliefs, which has been modified in unknown ways over the centuries, and which has no more accuracy in its depiction of the supernatural than the Iliad.
See also: “The Long, Strange Story of the Trinity.”

It ain’t supposed to make sense; it’s faith.
Faith is something that you believe
that nobody in his right mind would believe.
— Archie Bunker

Photo credit: Samuel Livingston