Actually that is a very good question. I never even thought to ask myself. I have heard some churches stipulate that if you don't believe in the trinity doctrine you cannot be a member. I am not sure I should really debate this because it doesn't say we must believe this in the Bible ; if fact, Jesus said let not your heart be trouble you believe in God believe also in me..."
I believe it is important to believe God is one an Jesus is His Word come in the flesh and that He died for our sin and rose. Although your question is a good one, why would I want to debate it when I said we are not taught to teach it? I believe God is Spirit and He is Holy. I don't mess with what makes my God tick. Muslims seem to use the trinity as leverage to win Christian people to Islam.