Why would John's Christian character be in question?

Hale questions Proctor on his churchgoing habits, and Proctor claims that he prays at home and criticizes Parris for his wasteful spending habits in church. Hale also notes that only two of Proctor's children are baptized, and asks Proctor to state the Ten Commandments. He names nine of them, but needs Elizabeth to remind him of the tenth – adultery. Proctor says that between the two of them they know all of the Commandments, but Hale says that no crack in the fortress of theology can be considered small.