achase79 said
In John 17:20ff, Jesus prays

²⁰ My prayer is not for them alone. I pray also for those who will believe in me through their message, ²¹ that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. ²² I have given them the glory that you gave me, that they may be one as we are one— ²³ I in them and you in me—so that they may be brought to complete unity. Then the world will know that you sent me and have loved them even as you have loved me

Jesus seems to think that this Christian unity is an important sign that is a precondition for the world believing in Jesus. He prays to the Father that it would be so. This obviously did not come to pass, with at least 41,000 distinct denominations as ** you do not have permission to see this link ** by the Center for the Study of Global Christianity at Gordon-Conwell Theological Seminary. Christians constantly argue among themselves, schisming and gatekeeping. This is a far cry from “complete unity.”

If I were to choose a religion base on the unity of it’s adherents (although I personally think it’s a poor criteria), I would probably be more inclined toward Islam (they have their sects, but as far as I know there seem to be fewer of them and they seem to be more consistent within their sects. Cf. the lists of ** you do not have permission to see this link **.)

Now, it might be fair to point out that perhaps Christian unity is out of Jesus’ or the Father’s control – after all, free will, right? (although these words don’t occur in the Bible, and although the idea of choice does occur, God is know to override free will, e.g. Pharoh). Maybe all those “so that”s (ἵνα) in John refer merely to intended purpose, not predicted outcome. But Jesus still asks the Father for it, which suggests that he thinks that the Father can make it happen. Jesus himself says (Matthew 7)

⁷ “Ask and it will be given to you; seek and you will find; knock and the door will be opened to you. ⁸ For everyone who asks receives; the one who seeks finds; and to the one who knocks, the door will be opened. ⁹ “Which of you, if your son asks for bread, will give him a stone? ¹⁰ Or if he asks for a fish, will give him a snake? ¹¹ If you, then, though you are evil, know how to give good gifts to your children, how much more will your Father in heaven give good gifts to those who ask him!

And if Jesus’ prayers can be ineffective, it’s not obvious to me why one would trust that Jesus is God, trust in Jesus’ other prayers to God, or believe, ala Rob Bell, that God ultimately gets his way.

Christians had to develop the fully man fully God thing to explain the contradictions.

Although it really doesn’t explain it away. Because if he is fully god and they say he as always been and always will be fully god, then he couldn’t have been tempted according to the Bible but the Bible says he was. If his fully man thing was the reason then he could not have been fully god.

One sect(Sunnism) in Islam is overwhelmingly dominant, and apart from the religious classes among Sunnis who argue certain creedal points with each other practically nobody else is interested in, Sunnis are largely identical.

 However I don’t think Christians see the differences as necessarily proof they are wrong. On the contrary they can use Biblical verses talking about false teachers and evil men with false teachings to explain sects that disagree with them.