Charles H. Spurgeon on Psalms 110:4

Sworn, and will not repent, Thou art a priest for ever. God might have made the levitical priest by oath, and yet he might have been changed, but if he had made him by oath to be a priest for ever, then he could not have repented, that is, changed; but he must of necessity have been a priest for ever. Therefore you must take special notice, that God did not only swear that Christ should be a priest, or that he should be a priest for a long time, but a priest "for ever; "so that there should never be any priest joined with him, or come after him. So that if we consider the oath, and the thing confirmed by this oath, two things will be manifest:

1. That Christ's priesthood is personal, and settled in one single person for ever; so that he can have no fellow nor copartner, nor any successor in his priesthood.

2. That, by this oath, God did limit his own supreme and absolute power in this particular; and took away the use and exercise of it, and that for ever.

For now he hath no power to make Christ no priest, or take away his priesthood at will and pleasure: and in this God discovered his unspeakable love unto Christ, in that he did so much honour him, and so highly reward him. By this he also displayed his abundant mercy to man; for by this oath known unto man, he signifies that man shall never be destitute of a powerful and effectual priest, able for ever to save; and this doth minister unto sinful man most sweet and heavenly comfort. George Lawson, 1662.