The principles by which we are to determine what element of the law enacted under the old dispensation is abrogated, and what element remains in full force under the new dispensation, are the following:
1) When the continued obligation of any commandment is asserted or practically recognized in the NT, it is plain that the change of dispensations has made no change in the law. Thus the provisions of the moral law are constantly recognized in the NT. On the other hand, when the enactment is explicitly repealed, or its abrogation implied by what is taught in the NT, the case is also made plain.
2) Where there is no direct information upon the question to be gathered from the NT, a careful examination of the reason of the law will afford us good ground of judgment as to its perpetuity. If the original reason for its enactment is universal and permanent, and the law has never been explicitly repealed, then the law abides in forced. If the reason of the law is transient, its binding force is transient also.
A.A. Hodge, The Confession of Faith (Banner of Truth 1983), 254-55.
The forensic or judicial law concerned the civil gov’t of the people of God under the OT and contained a body of precepts concerning the form of that political rule. There were various ends of it:
i) The good order (eutaxia) and legitimate constitution of the Jewish polity, which should be a true theocracy (theokratia), as Josephus calls it.
ii) The distinguishing of that state and nation from all other people and states and that that polity might be the seat of the church and the place for the manifestation of God.
iii) The vindication of the moral and ceremonial law from contempt, and so the enforcer of respect and obligation towards both.
iv) The adumbration of the spiritual kingdom of Christ.
In that law various ends must be distinguished. For inasmuch as it was a distinction of the Jewish state from the Gentiles and a type of the kingdom of Christ, it is simply abrogated because there is no longer any distinction between the Jews and the Gentiles in Christ (Gal 3:28; Eph 2:14). As the Jewish state and polity has been destroyed, so there is no need of a type to adumbrate the future kingdom of Christ, since it has already come.
But as to the good order (eutaxian) or form of gov’t of the Israelite people, it cannot be said to have been abrogated, unless relatively. Undoubtedly those things are to be accurately distinguished which in the law ere of particular right (which peculiarly applied to the Jews in relation to time, place and Jewish nation: such was the law concerning a husband’s brother, the writing of divorcement, the gleaning, &c.) from those which were of common and universal right, founded upon the law of nature common to all (such as the laws concerning trials and the punishment of crimes, widows, orphans, strangers and the like, which are of moral and common right.
As to the former, they may well be said to have been abrogated because the Jewish polity having been taken away, whatever had a peculiar relation to it must also necessarily have ceased. But ass to the latter, it still remains because it enters into the nature of the moral and perpetual law and was commanded to the Jews not as Jews simply, but as men subject with others to the law of nature.
For the distinguishing those things which are of common and particular right, a threefold criterion can be employed:
1) That what prevails not only among the Jews, but also among the Gentiles (following the light of right reason) is of common right. Thus the Greeks, Romans and others [e.g. Code of Hammurabi] had their own laws in which are many things agreeing with the divine laws (which even a comparison of the Mosaic and Roman law alone, instituted by various persons, teaches).
2) What is found to be conformed to the precepts of the Decalogue and serves to explain and conform it. This is easily gathered, if either the object and the matter of the laws or the causes of sanctioning them are attended to.
3) The things so repeated in the NT that their observance is commended to Christians.
Turretin, Institutes 2:165-68.