That's certainly all true. In addition, Proctor is also a voice of reason in The Crucible concerning Reverend Parris. Proctor is critical of Parris in nearly every way, showing his shortcomings about nearly everything. Proctor has a personal motivation, it's true; however, he points out Parris's greed in wanting golden candlesticks and more money for firewood. Proctor reminds him there's more to church and godliness than golden candlesticks. Proctor also points out Parris's lack of spiritual depth by criticizing his continuous sermonizing about hell and damnation. It is Proctor who offers another perspective on this supposedly godly man who is acting so self-righteously and pompously. Despite his own sins and faults, Proctor is able to keep things in balance for the audience and reveal the flaws in this man of God.
Proctor knows what's going on. If you carefully read Miller's diversions from his own play, he takes time to narrate the storyline behind the story, which is a land fight between neighbors. Proctor is described in his narrative as a man who is conscious of his own sin, but as with most Puritans dealt with the difficulty of having to be perfect. There seemed to be little acknowledgment of the parts of the bible that identify grace as a piece of Christian doctrine.
As time goes on, we learn his great sin was an affair with Abigail, but even as she tries to rekindle their romance, John remains faithful to the idea that he wants to respect his wife and make their marriage work by ensuring a clean break with Abigail.
Rebecca Nurse calls it like she sees it as a mother and grandmother several times over, she says Betty is faking. Proctor agrees.
After the Putnams convince Parris of witchcraft, John's reasonable voice notes:
Then let you come out and call them wrong. Did you consult the wardens before you called this minister to look for devils?
Proctor is pointing out that no one called the cops first to scare the girls out of this act, the adults present are just believing it to be witchcraft.
Proctor stands his ground throughout the Act and maintains his stance that the girls are faking.