I watch people use these interchangeably.
The problem is there are covenants (big ones, important ones) in the Old Testament not yet fulfilled (including everyone's favorite, the new covenant).
The old covenant, called the law, a country-establishing covenant between Israelites (of those present at Mt. Sinai) and God, was indeed fulfilled when the Husband (God) died in the flesh, releasing His Bride from the vow between Him and her and so releasing her from punishment of death, though she was adulterous and under the vow would have been put to death should the Husband still live. (I am quoting Paul. Romans 7. For some reason people still bristle at this.)
There are other covenants among those testified in the Old Testament that have also come to be fulfilled. An example is the covenant to have a descendant of David on an everlasting throne.
However, grasp that there are more covenants testified in the Old Testament that have not yet been fulfilled. One such *not-yet-fulfilled covenant of the Old Testament is the new covenant.* (Yes, the new covenant is from the Old Testament and is not yet consummated. It's found in Hosea 2 and Jeremiah 30, 31. For it not yet being consummated even after Christ's death and resurrection, Paul called the partial outpour of the Holy Spirit an earnest, a dowry. The consummation is called "the Last Day," "the Day of the Lord," "the Wedding Feast of the Lamb," so on.)
The new covenant is not the New Testament.
The new covenant is not from the New Testament.
There are no new covenants in the New Testament.
All covenants are found in the Old Testament.
The *fulfillment* of a given covenant however is found either in the Old Testament, in the New Testament, or has not yet come.
The New Testament only testifies of how Christ fulfilled and will fulfill specific covenants of the Old Testament.
The collection we call the Old Testament is "Old" not for being "all done" but for being chronologically first. It is a testament, not a covenant. It contains the covenants--even the new covenant.
Christ has yet to (but will) fulfill all unfulfilled covenants testified.
On the Last Day, upon the Wedding Feast of the Lamb with His Bride, all covenants not yet fulfilled will be, including the new covenant that will be consummated. There will be no more dowry only, but the full outpouring of the Spirit.
Until then, the new covenant is an Old Testament covenant among other Old Testament covenants not yet fulfilled.
The faith is that they will be.
