Explain the religious argument in An Essay on Man. What are some examples?

Pope's argument concerning religion in An Essay on Man is basically that people are not in a position to know enough about the universe to enable them to question God's plan and, still less, the actual existence of God. He uses the example of Isaac Newton—the Einstein or Stephen Hawking of his time—as one who, in spite of having explained the mechanistic laws of the universe, was still unable to answer the spiritual questions:

Could he, whose rules the rapid comet bind,Describe or fix one movement of his mind?

Man, in Pope's view, will never have the ability to explain why the universe came into being and what God's plan for him is.
A corollary of this idea is the much-caricatured view that this is "the best of all possible worlds," for which Pope's source is partly the philosophy of Leibniz. Man has no business judging God by the problems and imperfections that exist in the world because he has no way of knowing why God created the universe and man. Without imperfections, there would be no struggle and no problems to solve; these problems give "meaning" to life. To Pope, in these lines in which Newton is again referenced, man is to God and the angels as the animal kingdom is to man, the implication being that since animals don't understand man's purposes, neither should man presume to know God's intentions for us:

Superior beings, when of late they sawA mortal man unfold all Nature's law,Admired such wisdom in an earthly shape,And showed a Newton, as we show an ape.

Though Pope was nominally a Christian, the message is more that of deism, an ecumenical belief in a deity and in the equalty of faiths throughout the world.