No wonder that out text speaks of our need for genuine faith. Christian faith is like a mustard seed. If it is genuine, although it is small, it can do mighty things, not because of us but because of Christ. We sin much every day. Even Christians sin despite their best efforts.
Forgiveness of sins is a miracle.
It is like telling a tree to be uprooted and to be planted in the sea. That's where Christ our Savior and Substitute steps in. The whole Bible proclaims forgiveness of sins through Jesus Christ to everyone who believes it. I cannot rid myself of my sin. I cannot give myself a quiet conscience.