Psalm 110:4 is the key text that the author of Hebrews uses repeatedly to assure us of the eternal priesthood of Jesus. This time he emphasises that the priesthood of Jesus came with a divine oath. ″The Lord has sworn and will not change his mind: 'You are a priest for ever'″ (Hebrews 7:21). We saw in Hebrews 6:17 how God made an oath with regard to Abraham's descendants. That promise was eventually fulfilled in Christ, who more than Abraham could have imagined, made it possible for the spiritual offspring of Abraham to multiply into a vast number. This promise that God made to Abraham is connected with Jesus ″[becoming] a high priest for ever, in the order of Melchizedek″ (6:20).

This Melchizedek priesthood of Jesus is a certainty (Psalm 110:4). God has sworn it and will not change His mind about it; there is no other alternative and no other plan. Jesus is the High Priest for ever, and that will never change. Because of this divine oath, Jesus ″has become the guarantor of a better covenant″ (Hebrews 7:22). The Greek word enguos (″guarantor″, used only here in the New Testament) means ″surety″, one ″who guarantees that some undertaking will be honoured″.³⁴ The ″better covenant″ refers to the new covenant that Jesus established. At the Last Supper with His disciples, Jesus said, ″This cup is the new covenant in my blood, which is poured out for you″ (Luke 22:20). This new covenant replaces the old covenant that is based on the law of Moses and the temple sacrificial rituals performed by the Levitical priests. Now that Jesus has come, the old covenant is superseded by the new covenant, which is based on the once-for-all sacrifice of Jesus. In this new covenant, God offers us salvation freely in His grace.

We can imagine how this must have appealed to the Jewish Christians who were tempted, for various reasons, to leave the Christian faith to return to their Jewish faith. Why return to something that has already become redundant? Why leave the eternal solution from heaven for something that was only temporary? The Christian must remain with Christ who himself is the guarantee of the effectiveness of His new covenant.

³⁴Barclay, ″The Letter to the Hebrews″, 81.