When Jesus says, 'Your sins are forgiven', he's not just saying 'Your sins against me are forgiven' - he's saying 'ALL of your sins against ANYONE EVER are forgiven' or in other words, 'ALL your sins against GOD are forgiven' or to put it still another way, 'I (God) forgive you!'
When Jesus heard their thoughts, he knew they were right - it IS only God that can forgive a person's sin. The only one who can make a person right with God is God himself and Jesus is claiming to be able to do it and so he's basically claiming to be God.
Jesus knew the crowd would need more convincing about this (!) and so that's why he then heals the man physically (he'd already healed him spiritually but that's invisible). When the paralysed man gets up and walks for the first time in his life, the crowd realise several things beyond the shadow of a doubt (although I expect some were still managing to explain it away and complain about it as there always are)...
- A man's sin had been forgiven and he had been made right with God!
- A man who couldn't walk had been healed right in front of them!
- A man who they thought was just a man was saying and demonstrating that he was God himself!
- What's your reaction to Jesus' claim here to be God and to forgive sin and why?
- If you believe Jesus is God and can make people right with God, how much effort do you go to in bringing people to him? This paralysed man was lucky enough to have some gutsy friends who were willing to go to ridiculous lengths to present him to Jesus and this came before their concerns for themselves, for property and for social politeness! How gutsy are you in sharing your faith and why?