But Jesus knew what the Scribes were thinking too. They were thinking that Jesus was wrong to say this because only God could forgive sins. Jesus knows what you are thinking right now. He knows when you are thinking bad thoughts or mad thoughts or planning something selfish or mean. Now the Scribes were partly right.
Sin is very terrible.
It is when we do things that God has forbidden. Sin is also when we don't do what God has asked us. All sin makes God very sad. No matter how much good we do, God has to punish our sins. Only God can forgive sin.