The apostolic word establish is equivalent to Christ's word fulfill, in I came to fulfill. In the law there is, on the one hand, the demand of God's holiness and righteousness. On the other hand, there is an indication of how it must be fulfilled, according to the conditions and circumstances of the time.
The law of God would be destroyed if both were abolished. But if the demand remains the same, and a better path to its realization, or way of fulfilling it, is indicated, then the law cannot be considered abolished.
Its divine and eternal side remains the same, while only the temporary and, as it were, human side is changed. In this change, the demand of God's holiness and righteousness receives still better and more perfect satisfaction.
The law, being changed, is not destroyed, but established. Let us apply this to the Apostle's subject in Hebrews 7. The eternal demand of God's holiness consists in this: that there be priesthood and priests.
But besides this demand, the law of Moses also contained an indication of the way to realize it. This indication corresponded to the conditions of the Old Testament time.
It consisted in this: that the priests be from the tribe of Levi, from the house of Aaron, and that at their head stand the high priest, Aaron and his successors.
In the New Testament the eternal divine demand for the existence of priesthood remained the same. Therefore the law was not destroyed. But another, eternal High Priest arose, according to the order of Melchizedek, High Priest and King, see verses 1-4.
He has an unchangeable priesthood (v. 24), according to the power of an endless life (v. 16). He is holy, innocent of evil, separated from sinners, and exalted above the heavens (v. 26).
Has the law been destroyed? No, it has been established, since we have as High Priest the Son of God, perfected forever (v. 28).
In a similar way one must reason about all the other ordinances of the law of Moses. For example, the ordinances about sacrifices contained, on the one hand, the fundamental demand of God's holiness and righteousness that men purify themselves through sacrifices and offerings.
Yet God, as the all-sufficient Being, has no need of our good things, Ps 16:1. On the other hand, they contained particular indications for fulfilling this demand.
This was done through the slaughter of sacrificial animals on the altar of the tabernacle or temple, and through bringing into the tabernacle or temple the fruits of righteous labor (Prov 3:9).
In the New Testament, in fulfillment of this demand, the most perfect sacrifice of Christ has been offered and is offered in the Eucharist. Through this also the Old Testament law has not been destroyed, but fulfilled in the most perfect way, and therefore established.
Thus, in the light of the Word of God and under the guidance of Holy Tradition, one must distinguish what is imperishable in the Mosaic ordinance and what has ceased. One must distinguish what belonged to the Old Testament time and what was appointed to exist in all times.
THEOSIS