Zechariah 3:6 - Albert Barnes' Notes on the Bible

And the angel of the Lord protested - Solemnly (etymologically, called God to witness) as in, “Did I not make thee swear by the Lord and protested unto thee” 1 Kings 2:42, laying it as an obligation upon him. The charge is given to Joshua, and in him to all successive high priests, while Israel should continue to be God’s people, as the condition of their acceptance.