This question is presented to Jesus (Matthew 21;23-27); it is a testimony that those who ask it are hard of heart. Those who were docile recognized that it was by the power of God that Jesus spoke and healed.
This is the same authority that has established the Church and has given us the Sacraments to encounter Jesus. This is the same authority that says “your sins are forgiven” in Confession. It is the authority of God, in Christ that makes us whole if we are faithful to what He has revealed to us through the Scriptures and the Church.