In the biographical note about Proctor in Act I before Proctor ever speaks, Miller reports,
He is a sinner, a sinner not only against the moral fashion of the time, but against his own vision of decent conduct. These people had no ritual for the washing away of sins.
A few sentences later:
Proctor, respected and even feared in Salem, has come to regard himself as a kind of fraud. (pg. 19 in the Penguin Classics edition)
Each of these suggest that it was implicit in their culture that sin had no excuse. There was no way to get rid of it and Proctor wanted to be a virtuous man... other people believed as much. Thus, he is a victim of the Puritan religion which claims Christianity. A founding principle of Christianity is the forgiveness of sin, otherwise there would be no reason for Christ's death on a cross. But John bore his own sin.
These quotes above demonstrate direct characterization.
To see him indirectly characterized as one afraid of committing sin, look at these words to Abigail:
Abby, I may think of you softly from time to time. But I will cut off my hand before I'll ever reach for you again. Wipe it out of mind. We never touched, Abby. (pg. 22... Act I)
John says this to admit he has thoughts but doesn't want to allow them to turn into deeds. In fact, he is willing to punish himself if he does. He admits the sin and lies about it all in the same breath. This is interesting because he is using strong words, harsh words, but in his weakness does admit to the truth.