Matthew 9:2-8
2 And, behold, they brought to him a man sick of the palsy, lying on a bed: and Jesus seeing their faith said unto the sick of the palsy; Son, be of good cheer; thy sins be forgiven thee.
3 And, behold, certain of the scribes said within themselves, This man blasphemeth.
4 And Jesus knowing their thoughts said, Wherefore think ye evil in your hearts?
5 For whether is easier, to say, Thy sins be forgiven thee; or to say, Arise, and walk?
6 But that ye may know that the Son of man hath power on earth to forgive sins, (then saith he to the sick of the palsy,) Arise, take up thy bed, and go unto thine house.
7 And he arose, and departed to his house.
8 But when the multitudes saw it, they marvelled, and glorified God, which had given such power unto men.
In Matthew 9:2-8, Mark 2:1-12, and Luke 5:17-25, 7:36-50, Jesus forgives sins. Those present are recorded as being indignant, sometimes accusing Him of blasphemy, or asking, "Who can forgive sins, but God alone?" (Mark 2:7, Luke 5:21). Therefore, Jesus is being portrayed as God, and this is because He is God.
God is able to to delegate the authority to forgive sins to another person on His behalf, as Jesus Himself does with His disciples in John 20:23.
Additionally, Matthew's inclusion in Matthew 9:8 of the fact that "when the multitudes saw it, they marvelled, and glorified God, which had given such power unto men", is an indication that this is intended to be understood as a delegated authority. The same can be argued of Jesus's comment in Matthew 9:6 (Mark 2:10, Luke 5:24), where He says, "the Son of man has authority on earth to forgive sins". This statement, when examined in light of the many verses in which Jesus speaks of receiving power or authority from the Father (Matthew 11:27, 28:18, Luke 10:22, 22:29, John 5:22, 5:26-27, 5:30, 10:17-18, 17:2), most naturally appears to be a statement of Jesus saying that He has been given this power to forgive sins from God.
Therefore, those in the narrative who question, "Who can forgive sins, but God alone?", are mistaken in their presupposition, and Jesus corrects them in the context. Statements made by spectators in the Gospels need to be carefully examined, and cannot be automatically taken as what the authors themselves believe, as they are frequently incorrect, or miss the point (Matthew 13:54-58, Mark 3:21-22, Luke 7:39, 23:35-39, John 7:26-27, etc.). These authors do not believe that only God can forgive sins, because they believe that that authority has been delegated to the Son as well, which is the intended point of these narratives.