I don't think Christianity is a pyramid scheme because I know of know churches (with the exception of Jehova's Witness) that expect you to send in part of your income.

Besides, that's not how pyramid schemes work. You don't send money to the "top" of the pyramid, only the level above you.

I would say, however, that it is the nature of all religions to attempt to perpetuate their own existance, otherwise they would have died out again.