All of the major Abrahamic religions claim that if only all humanity would switch over to their brand of deliberate self-delusion then we would all wallow in peace and plenty, at least until the end of the world, which is nigh. I find that message difficult to reconcile with the very clear evidence that proseletyzing religions are in fact catastrophically divisive.
They all claim to have a book that is the exact and definitive word of "God". The trouble with that is that the books of Judaism, Islam and Christianity are all different. And the strongest adherents of these several books will enthusiastically kill one another to prove that their book is the authentic word of "God". One needn't read too deeply in history to see that not only is a Christian prepared to kill Jews and Muslims kill Christians and so forth, but Christians will energetically slay one another over questions of church organization and biblical interpretation. And Muslims have shown that they will with equal zeal shed blood over a hadith.
Even within a particular sect there is the question of who is in "God's" favor and who is not. If you are among the elect then it's hard not to view those who are in bad odor with the "Almighty" as people who are a little less than fully human and who should probably get a taste of what they're in for in the "afterlife". Since religions are the product of human minds it's no surprise that one person's notions of "God" and "godly" behavior is going to be different from another persons. And these differences, fortunately for non-believers, will constantly spin off heresies and reformations, to the detriment of the final immersion of humanity in the bliss of the true faith. And worst of all for the unification of humanity (which is a grim thought anyway) is the fact that organized religion is actually about gaining power over people. Priests, ministers, rabbis and mullahs use their "God" as a whip to cow their followers into obedience. And, of course, struggles between those seeking power are the most divisive of all.