How do we know the doctrine of Christ's priesthood is true?

The truth of Christ's priesthood is supported by His fulfillment of Old Testament prophecies and types. Hebrews 10 explains how the law was only a shadow of the good things to come, which were realized in Christ. The many sacrifices offered by the priests could never make the worshippers perfect, indicating the need for a superior sacrifice. Christ, as the one true high priest, fulfilled this need by offering Himself (Hebrews 10:11-12). His unique position as the sinless Son of God distinguishes Him from all other mediators and substantiates the claim of His eternal priesthood.