How do we know Jesus' authority to forgive sins is true?

The authority of Jesus to forgive sins is affirmed through His identity as the Son of God and the miracles He performed. In Mark 2:10, Jesus states that He has the power on earth to forgive sins, thereby affirming His divinity. The reaction of the scribes and Pharisees reveals their understanding that only God can forgive sins, which further establishes the unique authority of Christ. Additionally, His healing of the paralytic serves as a demonstration of that authority, as He not only forgives sins but also heals, showcasing His divine power to address both physical and spiritual ailments.