I am an unjust person. When I am wronged (at least in my opinion) I can just choose to forgive the person and not hold that "offense" against what I consider to be right, against the person. But what if I were a just person. Would it be just to forgive someone if the offense also hurt others. Say an atheist posts lies about Christianity. And those lies hindered someone from trusting in Jesus. Now if I forgive the person, am I not being unjust to the one who was led astray?
The Bible says only God can forgive sins, and I suspect there is a good reason for that. We use words like reconciled, redeemed, but what we are really saying is made righteous, debt free on the great scale of justice. Outside of Christ, our tangled web of unholiness precludes justice on that great scale, but being justified through the propitiation of Christ allows us to be shed of that body of sin, the so-called circumcision of Christ, done without hands.
Another neat thing about the mercy we obtain in Christ, not only our past and present sins are forgiven, but also our future sins. So the forgiveness is not tit for tat, not a forgiveness of specific sins, but an unconditional pardon, free at last, free at last, thank God almighty, we are free from the penalty of sin, now and forever. Praise be to God and our Lord Jesus Christ.