This is of course is a moment of high drama in Act II. As Hale questions John and Elizabeth, he makes it clear the way that any sign of weakness or omission of holiness can be regarded as a potential sign of witchcraft. Note what he says with unerring accuracy:
Theology, sir, is a fortress; no crack in a fortress may be accounted small.
What he is blind to is the way that John Proctor's omission of the sin of adultery is actually a perfect example of dramatic irony. We and Elizabeth and John both know that John has had an adulterous relationship with Abigail, and his inability to recite this commandment reflects his inability to own up and be honest about his own sin. Of course, this scene also foreshadows the time when John confesses his adultery openly in one last ditch attempt to prove that Abigail and the other girls are making up their stories. Thus this scene plays a very important part in terms of how it advances the plot of the play through its use of dramatic irony and how it foreshadows what is to come in Act III.