A very important chapter that you would benefit from studying in closer detail would be Chapter 11. This explains the gradual transformation in Dorian's character as he flirts with a variety of different forms of aestheticism and dandyism as he seeks to live his life by the ideals expressed in the book that Lord Henry lends him. It might be slightly harsh to suggest that he moves steadily towards satanism, and nowhere is this specifically mentioned in the book. Rather, this chapter makes it clear how in Dorian's mind his understanding of the difference between good and evil becomes perverted by the hedonism that is espoused in Lord Henry's book. Consider this telling comment that comes at the end of Chapter 11:
Dorian Gray had been poisoned by a book. There were moments when he looked on evil simply as a mode through which he could realize his conception of the beautiful.
Rather than refering to Dorian's moral decline as a slip into satanism, it would be more accurate therefore to describe his increasingly dilapidated spiritual state as a result of trying to pursue pleasure and beauty in their purest forms at the expense of everything else. Ultimately, it is the way that Dorian believes he is able to live his life without consequences that enables him to do this without the kind of moral compunctions that all of us have to live with.