Isn’t this a double standard? The Bible’s heroes took concubines and war captives as wives with no stated age limits, and Mary was likely a young teenager when betrothed. If “customs of the time” is no defense for Muhammad, it’s no defense for Scripture.

This is the objection a fair reader raises first. The Christian reply has two parts. The Bible does not present those practices as the timeless perfect model; it records them, regulates them, and in places condemns the figures involved, and Christians do not hold David or Moses to be sinless exemplars whose conduct binds all ages. The Islamic claim about Muhammad is specifically that he is the excellent example whose Sunnah is normative, which is what makes the consistency question sharper. Second, Christian ethics has hard texts of its own to reckon with, and they are not addressed here. Raising the question about one founder and not the other’s Scripture would be exactly the double standard you name.