What is the law during the time of Melchizedek?
The following interpretation is based on the 2012 NICNT commentary on the Epistle to the Hebrews by Gareth Lee Cockerill.
The pastor in the book of Hebrews understood Melchizedek of Genesis 14:17-24 as the "Priest of the Most High God" (Heb 7:3) who is greater than the Levitical priestly order (Heb 7:1-10). This pastor also interpreted Ps 110:4 as God's declaration of the Son's (Jesus Christ's) priesthood (Heb 5:5-6, 10; 6:20).
Therefore, it's quite straightforward that the law changed from the Mosaic Law to the New Covenant Law. We shouldn't see the law as going "back" because as the order of Melchizedek interpreted this way is eternal, the New Covenant law is eternal as well. Although the pastor's assertion would shock Jews such as Philo or the Pharisees (who see Mosaic law as eternal), the Hebrews pastor is teaching the other way around: that the Mosaic law has been fulfilled in Christ, and in fact the Mosaic law is foreshadowing Christ's New Covenant law, which is the one that is eternal.
From Gareth L. Cockerill's commentary on Heb 7:12:
... Life under the law was dependent on the functioning of the Aaronic priesthood. Therefore, “a change of the priesthood” is necessarily a “change of the law.”20 Thus the pastor would substantiate from the OT alone, without reference to Christ, that the prophesied change of priesthood anticipated a radical alteration in the relationship between the law and the people of God. This contention would shock those like Philo or the Pharisees who held that the law was eternally immutable21. And yet, as will become evident in the following verses, the pastor is convinced that fulfillment in Christ reveals what the law was always intended to be—not a means of approaching God but a God-instituted type and foreshadowing of the sufficient means of atonement and approach that would be provided by Christ.22
This interpretation is consistent with Aquinas's Summa theologiae, I-II, q. 107, a. 1: Is the New Law different from the Old Law. Aquinas reconciles Mosaic Law and New Covenant Law by explaining that they are the same in one respect (that the purpose is to help us to submit the same God) but different in another respect (Mosaic law was intended as teacher of children, while the New Covenant law as teacher of grown adults).
It seems that the New Law is not different from the Old Law:
But contrary to this: In Hebrews 7:12 the Apostle says, “When the priesthood is transformed, it is necessary for a transformation of the Law to be made.” But as the Apostle proves in the same place, the
priesthood of the New Law is different from the priesthood of the Old Law. Therefore, the Laws are likewise different.
I respond: As was explained above (q. 91, a. 4), all law directs human interaction in relation to some end. Now there are two ways in which things that are ordered to an end can be differentiated with
respect to the notion of an end. First, they can be differentiated by the fact that they are ordered to diverse ends, and this is a difference in species, especially if the end is a proximate one. Second, they can be differentiated by their closeness to or distance from the end itself. For instance, it is clear that two movements differ in species insofar as they are ordered to different termini; on the other hand, to the extent that one part of a given movement is closer to the terminus than another part, there is a difference
within the movement with respect to the perfect and the imperfect.
So, then, two laws can be differentiated in two ways.
In one way, they are differentiated in the sense of being wholly diverse, insofar as they are ordered toward diverse ends. In the case of cities, for instance, a law that was ordered toward rule by the
common people (populus) would be different in species from a law that was ordered toward rule by the aristocrats (optimates) in the city.
In the second way, two laws can be differentiated by the fact that the one of them orders things more closely to the end, while the other orders things more remotely. For instance, in one and the same
city, a law imposed on grown men (viri perfecti), who are capable of immediately doing what contributes to the common good, is different from a law meant to teach children, who have to be instructed in how to
perform the acts of men later in life.
Therefore, one should reply that, according to the first way of differentiating laws, the New Law is not different from the Old Law, since both have the same end, viz., that men should submit to God, and
there is just one God for both the New Covenant and the Old Covenant—this according to Romans 3:30 (“There is one God who justifies circumcision on the basis of faith and the lack of circumcision through
According to the second way of differentiating laws, the New Law is different from the Old Law. For the Old Law is, as it were, a teacher of children, as the Apostle says in Galatians 3:24, whereas the
New Law is a law of perfection, since it is a law of charity. On this score, the Apostle says in Colossians 3:14 that the New Law is a “bond of perfection.”
Would you agree that the law Abraham obeyed (Gen. 26:5) is already the “eternal” New Covenant Law? This is considering that Abraham and Melchizedek were contemporaries (Genesis 14).
Under this interpretation, everyone, not just Abraham, already had this law written in their conscience, which can also be called 'natural law' which God put as part of human nature created in the image of God. When Jesus came, this natural law was being made more explicit in his Sermon on the Mount as well as in other books of the NT, to which we name 'New Covenant Law'. The explication of 'natural law' in OT (Mosaic law) and NT (New Covenant Law) is necessary since human corruption clouds our conscience. Therefore, I think Gen 26:5 is referring to this 'natural law' (or we can say, 'creational ethic', see this article).
I don't think Gen 26:5 should be linked to Melchizedek. Abraham would see Melchizedek as a mysterious figure, identified as Christ only much later. It's possible that Melchizedek taught Abraham "my commandments, my statutes, and my laws.", but this is speculation. "Abraham obeyed my voice" most likely refers to Abraham's obeying the specific call for him (by faith) to leave Haran to go to Canaan, to accept the Abrahamic covenant, and to later sacrifice Isaac.