That friend has never seen the show, he reads the Manga. I felt that I could give us something to talk about besides school work so I decided to watch it. I liked it the minute that I started watching it, it's probably one of the greatest animes of all time.
Not only does it have subtle religious themes to guide the show but the story is very engaging and most of the characters (except maybe Kanda) are very likable. While I believe that Allen Walker is probably one of the coolest anime characters I have ever seen, some people may disagree and say others but this is only my opinion. Even if you don't believe that Allen Walker is one of the coolest characters ever you can still say that any of the Exorcists or main characters are very well drawn and thought out to the point where you start feeling for them because you are going through this journey with them to ...basically save the world.
I will give away no spoilers in this review because I believe that if I write a good enough review without revealing many important parts about the show then more people will want to see the show because they see that I am calling it one of the best Animes of all time. Of course, I do not watch anime all the time and I do not cos-play or watch any other anime, I have only truly devoted myself to this anime because of its truly engaging story, fight scenes, humor, characters, and theme songs (lol).
So if you like to be engaged while still laughing or even crying as well, then you will probably like D.Gray-man and I strongly encourage that you start watching it here on sidereel.