How do we know the doctrine of Christ's dual roles as king and priest is true?

In Zechariah 6:13, it is stated that 'He shall sit and rule upon His throne, and He shall be a priest upon His throne.' This dual role is unique because, unlike the Old Testament, where a king and priest could not fulfill the same position, Jesus embodies both by being our King who reigns and our Priest who offers sacrifices for sin. This is also supported by Hebrews 7:17, where it explains that Christ is a priest forever after the order of Melchizedek, validating His roles as king and priest in the heavenly realm.