Since there is no religion in the book, I assume you must be referring to the title. This title alludes to Biblical descriptions of the devil as coming in alluring and attractive disguises that are meant to deceive people as to his true nature. The devil, for instance, appears to Eve as a talking serpent who pretends to have her best interests at heart when he offers her the fruit from the tree of knowledge of good and evil. In the New Testament, in 2 Corinthians 11:14, we are told that:
even Satan disguises himself as an angel of light
Light is alluring and seems good. Likewise, Prada, a very expensive line of high status designer clothing is alluring and seems desirable. The title is saying that Miranda Priestly (and more largely, the fashion industry) appears disguised in beautiful and desirable clothing and wrappings (such as the white Hermes scarf Miranda always wears) but underneath is evil.
Other religious symbolism includes the name Priestley: Miranda is likened to the high priestess of an evil cult. Hermes is the Greek god of trade, commerce, and trickery (among other things) and by putting her in Hermes' scarves, Weisberger also associates her with these traits.
From the start, we see a world of false values, where worth is based on such externals as a person being very thin and very stylishly dressed, just as Miranda is. However, underneath her beauty, Miranda is a self-absorbed, entitled tyrant who runs her staff ragged fulfilling her whims, such as that her food always be hot at the moment she wants it, forcing her hapless assistant Andrea to constantly throw out food and run and buy it again so it will be warm when Miranda happens to come into the office.
The book—and its title—asks us to question the values of the fashion industry and the psychologically destructive way it is run. Main character Andrea learns she should not have bitten into this particular alluring "apple" of temptation, as she sacrifices her relationships and more or less ruins her life to service the "devil" who is Miranda. Luckily, she is able to break free of this devil at the end.