Luther on Deuteronomy 5:3 “Not with our fathers…”
Here Moses points out the difference between the New and the Old Testament.
The New Testament is the older, promised from the beginning of the world, yes, “before the times of the world,” as Paul says to Titus (1:2), but fulfilled only under Christ. The Old Testament promised under Moses was fulfilled under Joshua.
However, there is this difference between the two: the New is founded wholly on the promise of the merciful and faithful God, without our works; but the Old is founded also on our works. Therefore Moses does not promise beyond the extent to which they keep the statutes and judgments.
For this reason the Old Testament finally had to become antiquated and be put aside; it had to serve as a figure of that New and eternal Testament which began before the ages and will endure beyond the ages. The Old, however, began in time and after some time came to an end…Martin Luther (LW 9, Pg.63)