“‘the New Testament’ will appertain to none other than Him who promised it—if not “its letter, yet its spirit;” and herein will lie its newness. Indeed, He who had engraved its letter in stones is the same as He who had said of its spirit, “I will pour out of my Spirit upon all flesh.” Even if “the letter killeth, yet the Spirit giveth life;” and both belong to Him who says: “I kill, and I make alive; I wound, and I heal.” We have already made good the Creator’s claim to this twofold character of judgment and goodness—“killing in the letter” through the law, and “quickening in the Spirit” through the Gospel. Now these attributes, however different they be, cannot possibly make two gods; for they have already (in the prevenient dispensation of the Old Testament) been found to meet in One.” (ANF 3:452–53)

Tyconius writes in The Book of Rules:

The [Gospel’s] promise is distinct from the Law; and since they are different, they cannot be mixed. (under Rule III)

John Chrysostum writes in a sermon on 2 Corinthians 3:

In the Law, he that hath sin is punished; here, he that hath sins cometh and is baptized and is made righteous, and being made righteous, he liveth, being delivered from the death of sin. The Law, if it lay hold on a murderer, putteth him to death; the Gospel, if it lay hold on a murderer, enlighteneth, and giveth him life. (NPNF1 12:307)

Augustine writes in On the Spirit and the Letter:

His words are, “The righteousness of God is manifested:” [Romans 3:21] . . . This is witnessed by the law and the prophets; in other words, the law and the prophets each afford it testimony. The law, indeed, by issuing its commands and threats, and by justifying no man, sufficiently shows that it is by God’s gift, through the help of the Spirit, that a man is justified; and the prophets, because it was what they predicted that Christ at His coming accomplished. (NPNF1 5:88–89)