Hebrews 7:20

And inasmuch as it is not without the taking of an oath (for they indeed have been made priests without an oath; but he with an oath by him that saith of him, The Lord sware, and will not repent himself, Thou art a priest forever); by so much also hath Jesus become the surety of a better covenant.

The author still focuses on Psalms 110:4, shifting the emphasis to another strategic clause in it, "and will not repent," an expression not particularly noted until here. Milligan's exegesis on this is priceless. He said,

When God is said to repent, the meaning is that he simply wills a change; and when it is said that he will not repent, it means that HE WILL NEVER WILL A CHANGE (emphasis mine). And consequently, there is nothing beyond the priesthood of Christ to which it will ever give place, as a means of accomplishing God's benevolent purposes in the redemption of mankind.[24]

God, therefore, will never set aside the priesthood of Christ, as he did that of the Levites, the proof of this being that they were made priests without an oath of God, whereas Christ was made a priest forever after the order of Melchizedek, with an oath of God, and with the further promise that God will never repent of it, that is, never change Christ's priesthood as he did that of the Levites.

How is Jesus the surety of a better covenant? The entire gospel of his glorious life, together with all that he did and is doing, constitutes that surety. By his enduring the cross, rising from the dead, ascending on high, sitting down upon the very throne of God, and interceding for the redeemed - by these and many other things, Christ is the surety of the absolute truth and dependability of all that Christians have received concerning the new covenant and its inherent blessings. A similar thought is expressed in Acts 17:31, where it is related that God has given assurance unto all people in that he has raised Christ from the dead.

A comparison of the KJV and RSV texts on this place shows that "covenant" replaces "testament" in the later version. Perhaps covenant is the better word, but actually no word is capable of giving the full and total meaning of that system by which God will effect the redemption of humanity. As Milligan noted, even the word "covenant" has this objection, that it seems to imply some equality between the parties thereunto, which of course is not the case. No long discussion of the word "covenant" will be attempted here, but a few observations may be helpful. The best understanding of it lies in the fact that it is a "new" covenant, contrasting with the old covenant, and leading to the deduction that, as the totality of the Hebrew system, its laws, shadows, types, and ritual, was all summed up in the words "old covenant," then, in the same manner, "the new covenant" applies to all of the gospel with its ordinances, institutions, warnings, promises, hopes, and benefits, and which gospel exceeds and goes beyond the old covenant. The two covenants are alike in that both were given of God and bear upon the same invariable purpose of human redemption.