McComiskey: Christ as "promise-oath"
In speaking of Isaiah 49, McComiskey adds: "The supreme authority of the Old Testament is ultimately the same as that of the New Testament, that is Christ. It is Christ, not only in his objective delineation of Old Testament word, but in his living presence *in the word* (emp. mine; crb). He mediates the promises, according to Paul, not only in the New Testament era, but in the Old Testament era as well, for in some way those promises were made to Christ before the new covenant appeared in history...Christ became the facilitating principle of obedience... it was an objective realization of the relationship of Christ to the promise (in Deut. 30). This relationship permeates the covenants. It led ultimately to the appearance in history of the one to whom the words were spoken, 'I will keep you and will make you to be a covenant for the people....'(Isa. 49:8). -- Thomas McComiskey, The Covenants of Promise, pp. 227, 228.