Why do you think that religious belief and theology are always the result of indoctrination when there are clear counter-examples?
Just as there are handfuls of those raised in the Christian faith, who later converted to Islam.
The fact remains, that the odds are overwhlemingly in favor of the indoctrination of youth. If you were taught by your elders to be Islamic as a youth, then you will very likely embrace the Islamic faith. Likewise for the Christian faith.
Hence, the difference between the choice to embrace Islam or Christianity, is overwhelmingly correlated to the geographical accident of one's birthplace. And whether Christian or Islamic, the defenders of the respective faiths are equally convinced of the "truth" of what they were taught as children.
Why is this such a hard thing for you to admit?