In Against Heresies 4.16, Irenaeus distinguishes between the parts of the Mosaic legislation that are abrogated and those that are still in force. He has at least two divisions: one of temporary signs, and one of eternal applicability and validity.
Circumcision falls into the first group: “[W]e learn from Scripture itself, that God gave circumcision, not as the completer of righteousness, but as a sign, that the race of Abraham might continue recognisable….These things, then, were given for a sign; but the signs were not unsymbolical, that is, neither unmeaning nor to no purpose, inasmuch as they were given by a wise Artist; but the circumcision after the flesh typified that after the Spirit.” 1
As such, these signs had nothing to do with the justification of man before God, as Abraham proves, and not only Abraham, but also Lot, Noah, and Enoch:
And that man was not justified by these things, but that they were given as a sign to the people, this fact shows—that Abraham himself, without circumcision and without observance of Sabbaths, James 2:23 Then, again, Lot, without circumcision, was brought out from Sodom, receiving salvation from God. So also did Noah, pleasing God, although he was uncircumcised, receive the dimensions [of the ark], of the world of the second race [of men]. Enoch, too, pleasing God, without circumcision, discharged the office of God’s legate to the angels although he was a man, and was translated, and is preserved until now as a witness of the just judgment of God, because the angels when they had transgressed fell to the earth for judgment, but the man who pleased [God] was translated for salvation. Moreover, all the rest of the multitude of those righteous men who lived before Abraham, and of those patriarchs who preceded Moses, were justified independently of the things above mentioned, and without the law of Moses.
Before Abraham, and before Moses, Irenaeus says, the fathers “had the meaning of the Decalogue written in their hearts and souls, that is, they loved the God who made them, and did no injury to their neighbour.” However, over time, men forgot these laws: in Egypt, they fell “into oblivion.” As a result, God graciously made them known again in written form. The goals of the written Decalogue were the same as those of the unwritten Decalogue:
And it enjoined love to God, and taught just dealing towards our neighbour, that we should neither be unjust nor unworthy of God, who prepares man for His friendship through the medium of the Decalogue, and likewise for agreement with his neighbour—matters which did certainly profit man himself; God, however, standing in no need of anything from man.
God, of course, gave many other laws to the Israelites. Irenaeus, however, finds significance in the difference between the ways in which the Ten Commandments, on the one hand, and the rest of the Mosaic legislation, on the other, were delivered. First, the Decalogue:
Preparing man for this life, the Lord Himself did speak in His own person to all alike the words of the Decalogue; and therefore, in like manner, do they remain permanently with us, receiving by means of His advent in the flesh, extension and increase, but not abrogation.
The Lord spoke these words to Israel in propria persona. The rest of the legislation, in contrast, which is abrogated:
The laws of bondage, however, were one by one promulgated to the people by Moses, suited for their instruction or for their punishment, as Moses himself declared:And the Lord commanded me at that time to teach you statutes and judgments.
Note a couple of things here. First, all the laws are still the Word of God. But God gave one set–the eternal set–immediately, and the other set–the temporary set–mediately through Moses, the framer of the Israelite polity. These temporary laws were for a sign, and they are now cancelled in favor of what Irenaeus calls the “covenant of liberty”:
These things, therefore, which were given for bondage, and for a sign to them, He cancelled by the new covenant of liberty.
The others, however, continue on, and not only so, but increased and extended by the incarnate Jesus Himself. Irenaeus has already asserted this, but now expands upon it and identifies the Decalogue with the natural and common law:
But He has increased and widened those laws which are natural, and noble, and common to all, granting to men largely and without grudging, by means of adoption, to know God the Father, and to love Him with the whole heart, and to follow His word unswervingly, while they abstain not only from evil deeds, but even from the desire after them.
Why is that? So that they can now be justified by the keeping of the Law, in contradistinction to God’s people under the Old Covenant? Not so, says Irenaeus. The Law has a testing and evidentiary function. Obedience to it proves and provides evidence of faith for those who have “truly received the power of liberty”:
[All this is declared,] that we may know that we shall give account to God not of deeds only, as slaves, but even of words and thoughts, as those who have truly received the power of liberty, in which [condition] a man is more severely tested, whether he will reverence, and fear, and love the Lord. And for this reason Peter saysthat we have not liberty as a cloak of maliciousness,1 Peter 2:16 but as the means of testing and evidencing faith.