A popular phrase among Christians when describing the entire Bible is that “The OT is the NT concealed, and the NT is the OT revealed.”
Acknowledging this phrase means that one admits s/he is interpreting how the NT relates to the OT.
It means in some ways the OT & NT are the same, but in other ways “the two covenants,” as they are sometimes called, are different.
10 years ago, I believed and taught the word covenant was a better translation than the word testament, but now I realize how this contributed to my legalism back then.
By legalism, I mean a sincere attempt to keep all of God's commandments, but that unintentionally obligates God to save me, because I have kept all His commandments (in the NT). I now realize that my previous interpretation of how the OT & NT relate was that the NT was basically "Old Testament Light."
I believe that there are two major assumptions that contribute to this way of thinking.
One is that we are "under contract" with God and the second is that The Old Testament was nailed to the cross.
The OT was not "nailed to the cross" (Col. 2:14), so that the NT could be kept the exact same way with fewer laws to keep.
What was nailed to the cross was our sins or the record of our debt of sin to God. This is what the "handwriting of ordinances" and "written code" means. These phrases do not refer to the OT.
I believe translating the word "testament" as "covenant" is one reason some believe 5 steps and 5 acts must be kept meticulously in order to sustain this forgiveness, but that this actually hinders God's grace through an attempt to keep all of God's commandments in the NT.