"Faith" and "Commodus" are two terms that do not immediately bond to one another. If there can be something in which Commodus has faith, it would be his own ambition and his desire to be the next Roman emperor. Commodus says as much to his father, Marcus Aurelius. As it becomes painfully apparent that Commodus lacks what his father wishes in a son, Commodus suggests that he has "other virtues" such as ambition. This becomes telling because it is the last spoken moment of endearment towards his father.
After he commits patricide, Commodus seems to embrace the supposed "virtue" of ambition, almost as if it becomes a testament to his murdered father. Ambition becomes the embodiment of his faith. The more Commodus displays ambition, the more it seems to be something in which he believes. It moves to a point where Commodus is under the belief that the more ambition he displays, the greater the emperor he can be. It is in this where ambition and unchecked desire become the elements in which Commodus has faith and a sense of certainty.