Pope Shenouda III
The Reason Why God Alone Is the Forgiver of Sins:
(1) Forgiveness is an entitlement of God alone, because actually sin is committed against God. It is a breach of His commandments, a transgression of His law and a rebellion against His kingdom. It is also lack of love towards God, preferring evil to Him; it is ingratitude to Him. Sin is a rejection of God. This is clear from the words of the Lord: "I have nourished and brought up children, and they have rebelled against Me... They have provoked to anger the Holy One of Israel" (Is. 1:2-4).
(2) Even the sins which people commit against each other, before being sins against man, are foremost sins against God; against His commandments, against those whom He created. That is why David says in his Psalm of Repentance: "Against You, You only, have I sinned, and done this evil in Your sight" (Ps. 51:4). When Nathan faced him with his sin, he said: "'I have sinned against the Lord 'And Nathan said to David, 'The Lord also has put away your sin; you shall not die' " (2 Sam. 12:13,14).
See also (Mic. 7:9); (Is. 42:24); (1 Kin. 8:45,46) and (Deut. 1: 4 1).