The great doctors of Christian antiquity are quoted, and the Vatican II bishops add yet more weight to the value of lectionary reform:
God, the inspirer and author of both Testaments, wisely arranged that the New Testament be hidden in the Old and the Old be made manifest in the New. (St. Augustine, “Quest. in Hept.” 2,73: PL 34,623.) For, though Christ established the new covenant in His blood (see Luke 22:20; 1 Cor. 11:25), still the books of the Old Testament with all their parts, caught up into the proclamation of the Gospel, (St. Irenaeus, “Against Heretics” III, 21,3: PG 7,950; (Same as 25,1: Harvey 2, p. 115). St. Cyril of Jerusalem, “Catech.” 4,35; PG 33,497. Theodore of Mopsuestia, “In Soph.” 1,4-6: PG 66, 452D-453A.) acquire and show forth their full meaning in the New Testament (see Matt. 5:17; Luke 24:27; Rom. 16:25-26; 2 Cor. 14:16) and in turn shed light on it and explain it.
It’s not a hard principle, actually, given that the whole of God’s activity in the human sphere has been and is directed at our salvation.