Why do church doctrines differ when there is only one Bible so should be only one belief?
The reality of multiple denominations and beliefs within Christendom is a consequence of two forces: the enemy (Satan) and humanity's sinful flesh. The enemy is always at work creating deception and seeking to confuse and mislead believers. Jesus calls Satan the father of lies and says the truth is not in him.
Therefore, over the course of time the enemy has sown seeds of deception by introducing false teaching, and many Christians have followed after his lies rather than listening to the Spirit as He explains the Word of God. When division arises, unity is lost and denominations arise.
Lack of unity is a result of the second negative influence: our flesh. Our sinful nature is prideful, stubborn, selfish and generally evil, and so our flesh leads us to favor sinful priorities above pursuit of the truth. Often, we would rather be "right" than know the truth, and we often don't recognize the problem in ourselves.
Nevertheless, the Lord has the power to reveal Himself clearly to us, and since we don't have uniform understanding, it's clear He has chosen to allow disagreements and differences of opinion to exist in His body as a trial or test of our willingness to hear and obey Him. Not all who receive the Word of God will do the right things with it, and so mankind's differences of understanding reveal a difference in humility and dedication to learning the Word. The Lord will use these differences to judge among His people.
And then in a day to come, the Lord will give us complete understanding, as Paul says:
1Cor. 13:9 For we know in part and we prophesy in part;
1Cor. 13:10 but when the perfect comes, the partial will be done away.
In the meantime, differences in understanding remain even though the Word of God tells only one story.
