“Your sins are forgiven” is in the passive voice, which in Jewish idiom means God has forgiven them. Priests declared forgiveness after sacrifices. How is this a claim to be God?

The divine passive is a real feature of Jewish speech, and if Jesus had stopped there the objection would be strong. The scribes, who knew the idiom, did not hear it that way. They heard blasphemy, because forgiveness was being pronounced without sacrifice, without the temple, and on Jesus’s own initiative. Jesus then heals the man so that you may know that the Son of Man has authority on earth to forgive sins, which claims the authority for himself rather than deflecting to God. A Qumran text (the Prayer of Nabonidus) has a Jewish exorcist forgive a king’s sins, so the category of a human agent of forgiveness existed; what is striking is that the scribes in Mark did not place Jesus in it.