Matt. 5:17

The perfection of the law is proved.

IV. The arguments of the orthodox are: First, “the law is perfect” (Ps. 19:7), not only relatively and with regard to its age under the Old Testament, but also absolutely with regard to nature; and so perfect as not only to contain all things to be done, and there is no defect in these (Genebrardus, Psalmi Davidis [1606], p. 71 on Ps. 18 [19]:9, and Lorinus testifying on this passage), but also to be capable of no addition to or detraction from it (according to Dt. 4:2; 12:32). (2) It is perfect extensively as to parts because it adequately embraces in the love of God and our neighbor all that is due to them (as Christ teaches in Mt. 22:37); intensively as to degrees because it demands not any, but the most perfect love and a greater than which cannot be conceived; and finally as to perfect use and effect because it can grant to its observers life and happiness—“If a man do these things, he shall live in them” (Lev. 18:5). This could not be said if they needed either addition or correction.
V. Second, Christ “came not to destroy but to fulfil the law” (Mt. 5:17). Here plērōsai does not mean to make the imperfect perfect or to correct the faulty (which would rather be to destroy [analysai] it), but according to Hebrew usage to do what is commanded. Thus Christ fulfilled the law not by addition or correction, but by observation and execution. He fulfilled the law in three ways: either as a doctrine by faithful preaching, solid confirmation and powerful vindication; or as a rule (normam) by a full and consistent keeping of it; or as a type by a perfect consummation, by exhibiting in himself the truth of the types and prophecies and the body of its shadows.
VI. Third, Christ in the New Testament did not introduce (either by himself or by his apostles) any other precepts of the law than those which had been given by Moses (Mt. 22:37; Rom. 13:9). He gave no other explanation of them than had been given before by the prophets. Hence the commandment of love is called by John “old and new” (1 Jn. 2:7, 8): old with regard to its first promulgation in the Old Testament; new with regard to its renovation and illustration in the New.
VII. Fourth, the law could not be supplemented or corrected without convicting it of imperfection and faultiness (and so charging that faultiness upon God, the author of the law). Even the thought of this is impious.
VIII. Fifth, the “denial of ourselves,” “bearing the cross” and “imitation of Christ” were already commanded under the Old Testament. For when we are entreated to love God above all things are we not by that very command charged to deny ourselves for God’s sake and to patiently bear the cross imposed upon us by him? And when the imitation of God is so frequently prescribed, is not the imitation of Christ (who is true God) also commanded by it? Besides, the imitation of Christ lies in the practice of the moral virtues, the rule for which is to be sought only in the law. This is also confirmed by various examples in the Old Testament: as to denial—by Abraham leaving his country and getting ready to sacrifice his son; the Levites slaying their brothers (Ex. 32); Job blessing God in adversity (Job 1:21, 22); Moses, Daniel and his friends disdaining all pleasures (Heb. 11:25, 26; Dan. 1:8ff.). Note also the following: as to “bearing the cross”—by Moses (Heb. 11:25, 26), the prophets under Ahab (1 K. 18:4), Zechariah (2 Chron. 24:20–21), the friends of Daniel (Dan. 3), the Jewish church under Antiochus (Ps. 44; Heb. 11:33–38). As to the “imitation of Christ,” by believers conducting themselves after the example of God.
Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 2 (Phillipsburg, NJ: P&R Publishing, 1992–1997), 20–21.