God alone can forgive sin. For only the one against whom an offense is directed can forgive the offense. Sin is imputed to man as an offense not only by another man, but also by God, as we said above. However, we are now considering sin as imputed to man by God. Accordingly, God alone can forgive sin.
Again, since by sin man is deflected from his last end, sins cannot be forgiven unless man is again rightly ordered to his end. This is accomplished through the gifts of grace which come from God alone, since they transcend the power of nature. Therefore only God can remit sin.
Further, sin is imputed to man as an offense because it is voluntary. But only God can effect a change in the will. Consequently He alone can truly forgive sins.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Remission of Sin by God Alone, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).