The idea that Moses returns to take the animals to the promised land is ironic in Animal Farm. The allusion to the religious/historical Moses is meant to show how organized religion is a distraction from the harsh realities of real life. When Moses returns, the pigs denounce his claims about the Sugarcandy Mountain. But they allow him to continue his preaching because it does give the animals some hope that even if they work like slaves in this life, they can at least think about a better existence in the afterlife. Moses' return (and the fact that the pigs allow him to preach) is an allegory about Karl Marx's famous quote that religion is an opiate for the masses. What he meant was that religion could be used to pacify people, to get them to think of their difficult lives as a necessary struggle towards a greater reward in heaven.
In Chapter 9, the animals now want to believe Moses more than ever before. "Their lives now, they reasoned, were hungry and laborious; was it not right and just that a better world should exist somewhere else?" (45).
Animal Farm is an analogy of the Russian Revolution. Stalin closed many of the churches. The ones he left open, and in some cases reopened, were strictly controlled by the State and used to promote the State's propaganda (just as Moses is allowed by the pigs to stay).