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.
When John Proctor is asked by Reverend Hale to recite the Ten Commandments he is ironically unable to name off the commandment regarding adultery. This is significant because Proctor carried out an affair with Abigail Williams, and although he and his wife Elizabeth were seemingly able to move on and come to terms with this indiscretion, the lengthy moment where we realize that he cannot identify his biggest sin is incredibly heavy.
The commandment against adultery- which is the one he has "forgotten" in real life as well. He has committed adultery with Abigail Williams, and that commandment is the one he can't remember. It is a nice piece of irony.
When Reverend Hale asks Proctor to recite the ten commandments, Proctor ironically omits the commandment that forbids adultery. This omission is significant due to Proctor's affair with Abigail.
When Proctor recites the Ten Commandments, he purposefully and ironically omitted the commandment that has forbidden adultery and condemned it as a serious crime that is punishable. He didn't name it as it makes him faintly as he would remember the time when he and Abigail committed adultery and developed an affair. He exposes his deficiency of the Christian morality but also suggest a notion that his entire household had been succumbed to witchcraft and was under Abigail tight control and evil influence.