Why was the name Old Testament first used by Melito of Sardis?

The term "Old Testament" was coined by Melito of Sardis in the 2nd century CE.1

In the context of early Christian theology, the name was used to distinguish the Hebrew Bible—which Christians viewed as the record of God’s covenant with the people of Israel—from the emerging collection of Christian writings that would become the New Testament.12

The Greek word diathēkē, which is translated as "testament," also carries the meaning of "covenant". By adopting this terminology, early Christians sought to frame the Hebrew Scriptures as the "old" covenant, which they believed was fulfilled by the "new" covenant established through Jesus Christ. This conceptual shift helped the early church legitimize the gospel as the natural extension of the Abrahamic covenant while asserting the continuity of divine revelation.21