Adam Clarke on Luke 5:21

Who can forgive sins, but God alone? - If Jesus were not God, he could not forgive sins; and his arrogating this authority would have been blasphemy against God, in the most proper sense of the word. That these scribes and Pharisees might have the fullest proof of his Godhead, he works in their presence three miracles, which from their nature could only be effected by an omniscient and omnipotent Being. The miracles are:

1. The remission of the poor man's sins.

2. The discernment of the secret thoughts of the scribes.

3. The restoration of the paralytic in an instant to perfect soundness.

See on Mat 9:5, Mat 9:6 (note).