Hale, concerned that the Proctors have been mentioned briefly in court, goes to their house in order to get to know them a bit better. He wants to get a feel for their faith, and question some of their weak spots a bit, to see if there is any possible validity to potential claims of witchcraft that might come in regards to the Proctors.
There are, unfortunately, weak spots in the Proctors' spiritual records. For one, they haven't been to church much in the past year, AND, their recently born third son hadn't been baptized yet. No baptism for an infant meant that the child could not be saved in heaven. Proctor explains that it was because Elizabeth "were sick" quite a bit, which made it difficult to go to church. Also, to be honest, they really don't like Reverend Parris, so didn't want him baptizing their son.
Hale takes this information in, and as a further "test," asks John to recite his commandments. The commandments, in Puritan communities, were law. And, if you recall earlier in the act, Goody Osburn was convicted of being a witch, in part because she couldn't recite her commandments. John proceeds, but ironically forgets the commandment that states, "Thou shalt not commit adultery." This is ironic because that is the commandment that he has recently broken, by having an affair with Abigail. You would expect him to remember that commandment, since he was such a violator of it in the past. But, ironically, he doesn't, and his wife chips in with that one. John tries to play it lightly, telling Hale that
"between the two of us we do know them all. I think it be a small fault."
Hale counters by saying that it is the smallest cracks in a fortress that bring the walls down. I hope that clarifies things for you a bit; good luck!