When Reverend Hale visits the Proctors' home, he says it is to "put some questions as to the Christian character of this house," meaning, essentially, that he means to quiz them in order to find out if they are true followers of the faith. John Proctor's religious feelings are already called into question by the fact that he does not attend church regularly, as he claims to be able to pray just as well from his own house, and because he is sickened by the Reverend Parris' pleading for money from the pulpit. His religion is also called into question because he has not had his children baptized, saying that he does not see the light of God in Parris, which shocks Hale.
When Hale asks Proctor to recite the Ten Commandments, which, in his mind, any true and abiding Christian should be able to do, John recites them fairly easily until he gets to the end. He omits the eighth commandment ("thou shalt not commit adultery") but repeats the third commandment ("thou shalt not make unto thee any graven image") twice. This omission is poignant because John Proctor broke the eighth commandment by sleeping with Abigail Williams, who used to work for them, and who is the very person orchestrating the witch hunt in Salem. To make the situation worse, Proctor's wife, Elizabeth, is standing by as he tries to recite his commandments, and she ends up correcting her husband when Hale points out John's mistake. Elizabeth Proctor is fully aware of her husband's indiscretion, which becomes an important factor later in the play when John and Elizabeth are trying to prove their innocence in court.