Psalms 110:4

4 Jehovah hath sworn This verse is a satisfactory proof that the person here spoken of is none other than Christ. When the Jews, with the view of mystifying this prediction, render the term כוהן, chohen, a prince, their translation is at once feeble and frivolous. I acknowledge, indeed, that those of noble descent or of royal blood are in Hebrew denominated כהנים, chohanim; but would it have been saying any thing to the honor of Christ for David merely to give to him the title of a chief, which is inferior to that of royal dignity? Besides, what would be the import of saying that he was a prince for ever, and according to the manner of Melchizedek? There can be no question then, that the Holy Ghost here refers to something specific and peculiar, as distinguishing and separating this king from all other kings. This, too, is the well known title with which Melchizedek was honored by Moses, (Genesis 14:18) I grant, indeed, that anciently among heathen nations kings were wont to exercise the priestly office; but Melchizedek is called “the priest of the most high God,” in consequence of his devoutly worshipping the only true God. Among his own people, however, God did not permit the blending of these offices. Hence Uzziah, David’s legitimate successor, was struck with leprosy because he attempted to offer incense to God, (2 Chronicles 26:21.) The circumstances connected with the lineage of David were vastly different from those relating to Melchizedek. What these are it is not difficult to ascertain, inasmuch as in this new King the holy office of the priesthood shall be united with the crown and the throne. For assuredly the imperial majesty was not so conspicuous in such an obscure prince as Melchizedek, as on that account to warrant his being held out as an example above all others. Salem, the sole seat of his throne, and where he reigned by sufferance, was at that time a small obscure town, so that with regard to him there was nothing deserving of notice saving the conjunction of the crown and the priesthood. Ambitious of procuring greater reverence for their persons, heathen kings aspired after the honor of the sacerdotal office; but it was by divine authority that Melchizedek was invested with both these functions.

All dubiety as to this being the meaning of David ought to be banished from our minds by the authority of the Apostle. And although the Jews may maintain the contrary as obstinately as they please, yet reason manifestly declares that the beauty of holiness, to which I formerly adverted, is here very clearly described. To this a decisive and peculiar mark is appended, which elevates Christ above all other kings with regard to the dignity of the priesthood, and which at the same time tends to point out the difference between his priesthood and that of Levi. In connection with his sacerdotal office, mention is made of God’s oath, who was not wont to mingle his venerable name with matters of minor importance; but, on the contrary, to teach us by his own example to swear deliberately and reverently, and never unless in weighty and important matters. Admitting, then, that God had sworn that the Messiah would be the prince and governor of his people, according as Melchizedek was, this would have been nothing else than an unbecoming profanation of his name. When, however, it is quite apparent that something unusual and peculiar was denoted in this place, we may therefore conclude that the priesthood of Christ is invested with great importance, seeing that it is ratified by the oath of God. And, in fact, it is the very turning point upon which our salvation depends; because, but for our reliance on Christ our Mediator, we would be all debarred from entering into God’s presence. In prayer, too, nothing is more needful for us than sure confidence in God, and therefore he not only invites us to come to him, but also by an oath hath appointed an advocate for the purpose of obtaining acceptance for us in his sight. As for those who shut the door against themselves, they subject themselves to the guilt of impeaching him with being a God of untruth and of perjury. It is in this way that the Apostle argues the disannulling of the Levitical priesthood; because, while that remained entire, God would not have sworn that there should be a new order of priesthood unless some change had been contemplated. What is more, when he promises a new priest, it is certain that he would be one who would be superior to all others, and would also abolish the then existing order.