Before anyone explodes about there being more than two options, yes, I know. Not the point.
Both Christianity and Islam have their Abrahamic roots, but clearly, both oppose each other in terms of belief systems. Neither can be right at the same time, so clearly, at least one of these religions is wrong...the alarming fact being that billions of people will suffer the torments of Hell regardless of which side is right.
Christianity requires obedience to and acceptance of Jesus Christ, as told in the Bible, whereas Islam rejects Jesus being the son of God, and instead allows salvation through the laws and ideology of Islam. So, clearly, if one is right, the other is is wrong.
So, how do you each justify your own religion when the opposing side is every bit as convinced as your own, and how do you find justification in your religion if your deity allowed such a perversion from the origins of the religion to the point that billions of people will be suffering in Hell because of the allowed confusion between them?
And yes, I could include Judaism, since it's the source of both, but their ideology doesn't condemn anyone in the way that Christianity and Islam do.