As one who is conscious of his own sinfullness, I agree that one can be a believer and a sinner. In fact, I agree that all have sinned, believer and non-believer alike.
An observant Jew (or Christian or Muslim) might give into the the temptation to steal, and then repent. If, however, a person is continuing to choose a life based on a practice that is repugnant to a religion, the person can't then claim to follow that religion.
Fagin was not a man who slipped and gave into the temptation to steal. He was a unrepenant professional thief.
You can be a gigolo. You can be Amish. You can't be an Amish gigolo.
I am not Roman Catholic, but I strongly disagree that unrepenant murderers or serial child rapists can be described as "devout" Roman Catholics. They may be lapsed, or fallen, or former, or false, or self-proclaimed, or maybe even "culturally" Roman Catholic, but they are not religiously Roman Catholic.