The devil is really in the details in this one, much more so than most would think. I'll skip ahead a bit and say I love this movie as a whole.
But it's best to examine the core first. Which is a small time lawyer with an absolute A+ winning record gets offered and accepts an offer for the big leagues in the world of lawyers. It's obvious the clients he's being brought are guilty, but his job is to garner a not guilty verdict, which he does every time without fail. All the while he has an on and off sorta relationship with his wife going. He loves her, but the on and off comes from being torn between her and work. His devout Christian mother senses what's going on all along and makes no secret of it. She predictably knows much more than she lets onto. Towards the end his wife realizes he's in fact working for the devil and commits suicide. After which he seeks out his mom and demands answers which he knows she has. Then the climax begins, which is the ending. He meets his boss and confronts him that he's onto him.
Ahh, here's where it truly gets good. Al Pacino portrays the devil brilliantly, but also in an appealing manner. He knows he's doomed! So why not embrace it?! Why not indulge in it?! Already condemned to a life in hell how can it get any worse?! Literally have the absolute worst possible outcome 100% guaranteed to happen!!! So let's embrace it all and enjoy it. Besides, as the movie so points out, naturally their fate is eternal damnation, look who wrote it!
This is the movies message. Get how there's much more to this than the surface? Here we have an absolutely brilliant movie with an all star cast brilliantly acted and all to show the glamour & glitz that is Satan, or so this movie means to make him appear. Way to go Hollywood, you've effectively glamourized the devil and made his ways appear very appealing. The devil is a master of deception, but here we have this aided by Hollywood's script. Perhaps some truth to it, but it's Hollywood's script that makes it appear appealing. It's one thing to make an entertaining movie about the devil, it's quite another to make him out to be the male equivalent of a Hollywood showgirl as a lawyer.