This can be stated in active form. AT: "You should not argue about this in your hearts." or "You should not doubt that I have the authority to forgive sins." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Jesus uses this question to make the scribes think about what might prove whether or not he could really forgive sins. AT: "I just said 'Your sins are forgiven you.' You may think that it is harder to say 'Get up and walk,' because the proof of whether or not I can heal the man will be shown by whether or not he gets up and walks." or "You may think that it is easier to say 'Your sins are forgiven' than it is to say 'Get up and walk.'" (See: rc://en/ta/man/translate/figs-rquestion)
The unspoken implication is that one thing is "easier to say because no one will know," but the other thing is "harder to say because everyone will know." People could not see if the mans sins were forgiven, but they would all know if he would get up and walk. (See: [[rc://en/ta/man/translate/figs-ellipsis]])