We accept man's testimony, but God's testimony is greater because it is the testimony of God, which he has given about his Son. Ninety-nine percent of the things you believe are believed on authority. I believe there is such a place as New York. I could not prove by abstract reasoning that there is such a place. I believe it because reliable people have told me so.
Every historical statement is believed on authority.
None of us has seen the Norman Conquest or the defeat of the Spanish Armada. A person who balked at authority in other things, as some people do in religion, would have to be content to know nothing all in this life. Lewis God gave his testimony about Christ. The Law of Moses required two or three witnesses to establish testimony as true. God lived by his own law as He gave the witness of the water, blood and Spirit.