It’s a little more complicated than that, actually. It’s not that Christianity is unwilling to “to assert the limits of behavior.” On the contrary, Christianity has very definite boundaries on acceptable behavior. But you have to understand the difference between – as Christians put it – Law and Gospel. “Law” is the Law of Moses, the rules that must be obeyed and the breaking of which condemns you to hell. “Gospel” is the news of Christ’s fulfillment of that Law and the knowledge that you no longer have to live a perfect life in order to be saved.
Christianity preaches both, with the emphasis on the Gospel. Christianity is difficult because people prefer definite, safe, prescriptions for behavior – do this deed, or refrain from this activity and you will be saved. Christians are not saved by adherence to the law, they are saved by faith – which can be a very difficult thing. (In Christianity, the deeds follow from the faith. “You will know them by their fruit.”)
Islam, on the other hand, is pure Law. If you follow the 5 pillars of the faith you are acceptable to Allah. There is no thought required, no moral judgment. You can even live as evil a life as you please – just so long as you don’t break the Law as listed by Muhammad. It’s easy, just so long as you don’t think too hard.
