OD forgives sins. This is marvelous. He is not required to forgive. There are no external forces that demand His grace. He freely gives it. This is good news indeed. It is wonderful enough that God has the power to heal our bodies, but His ability to heal our souls – this wonder exceeds all others. And, the content in Matthew 9 illustrates God's authority over all the effects of the fall: spiritual and physical.
Imagine meeting Christ in person and hearing Him say, "Son, be of good cheer; your sins are forgiven" (Matthew 9:2). Jesus did stuff like that. And, He had a right to do it too. The crowd who saw Jesus (on the day when he made that very statement) was amazed at His power over physical problems (Matthew 9:8), but unfortunately, they missed the bigger picture.
Matthew didn't miss it though. When Jesus called on him to leave his lucrative career as a tax collector, he did just that (Matthew 9:8). This decision was a spiritual decision. Even with the visible power which Christ was demonstrating in those days, Mathew would surely have had no inclination of leaving his station for purely material motives. There was a spiritual work going on in his heart. The fruition of that reality is seen in the eventual apostleship of Matthew and in the writing of this very gospel account.