Q. Doesn’t 1 John 9 say that “if we confess our sins He (God) will forgive us our sins and purify us from all unrightiousness” mean that when we confess, God is forgiving us not the person we confess to,
A. Yes, of course, it absolutely is God who forgives through His minister, the priest. The priest is His representative and provides audible assurance of forgiveness, and counsel.
In fact, if someone has the wrong heart attitude and is able to fool the priest that he is repentant when he is not and has no intention to avoid the sin in the future—God does not forgive the sin in actuality. So, even though the penitent hears the words of absolution, God is not mocked. Also, if he withholds confessing a deadly sin out of embarrassment, the confession of other sins and the absolution are worthless. Again, God is not mocked. If a deadly sin is truly forgotten during confession, it needs to be confessed next time but it does not invalidate the confession.
Q. and even if we confess to God privately.
A. But we are required to go to confession for deadly, as in grave, sin. Venial sins we may confess and be forgiven privately by praying to God on our own.
Q. So the Catholic Priest can say with Biblical authority that your sins have been forgiven without some special positional statement stemming from Peter.
A. Yes, however, Iit is not “a positional statement from Peter” but stated by Jesus Christ Himself, as recorded by the Apostle in Sacred Scripture, John 20. Now Protestant will interpret this verse differently. Protestant Interpretation vs Catholic Interpretation. Both are valid interpretations as interpretations go.
But how can the truth, the true interpretation be found? I am convinced that the only way is to go outside of the Bible and find out what the earliest Christians believed and practiced in relation to this verse. They Practiced oral, audible confession to the whole community at first and later made it easier on weak souls by allowing confession to be private between the penitent and the priest.