Indoctrination explains why religions spread. Most religious people are the nominal ones, not the converts, religious officials, or fundamentalists.

Christianity may not be pure fantasy, but it's much more fantasy-driven than is a nontheistic worldview. If you asked people what world they'd prefer to be real, a monotheistic one or atheistic naturalism, I think they'd choose the former. Even most atheists would prefer at least the chance, if not the guarantee, of immortality, perfect justice, and of final answers about the meaning of life.

I think it's ludicrous to say that Christianity imposes harsh restrictions on its followers. Priests and monks may have restricted lives, but ordinary Christians can do whatever they want and rationalize it. Look at Evangelical Christians' embrace of Trump, or the prosperity gospel. Look at Catholicism and the Crusades and Inquisition. So that's typical snide nonsense from CS Lewis.

The key point is that this choice between worldviews doesn't occur in a vacuum. Even if Christianity's not pure fantasy in every respect (like the grotesque car that Homer Simpson designed when he had the chance, in that one episode), that religion is still more intuitive, comforting, childlike, and anthropocentric than is atheistic naturalism. Christianity flatters us more, whereas naturalism puts us in our place, removing us from the center of things.