Wherefore it behoved him a in all things to be made like to [his] brethren, that he might be a merciful and faithful high priest in things relating to God, to make propitiation for the sins of the people;
He speaks, I apprehend, historically; it was necessary for him to do this by the alleged reason, not his present judgment of divine necessity or purpose. 'Has behoved' would speak more of continuance. It behoved him when he became a man. It is what he became as man, not what he took on him.