How is John Proctor a voice of reason in The Crucible by Arthur Miller, in Act 1?

That's
certainly all true.  In addition, Proctor is also a voice of reason in
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.