The plan of salvation is presented under the form of a covenant. This is evident:

First, from the constant use of the words berit and diatheke in reference to it.

With regard to the former of these words, although it is sometimes used for a law, disposition, or arrangement in general, where the elements of a covenant strictly speaking are absent, yet there can be no doubt that according to its prevailing usage in the Old Testament, it means a mutual contract between two or more parties.

It is very often used of compacts between individuals, and especially between kings and rulers. Abraham and Abimelech made a covenant. (Gen. 21.27) Joshua made a covenant With the people. (Josh. 24.25.) Jonathan and David. made a covenant. ( 1Sam. 18.3) Jonathan made a covenant with the house of David. (1 Sam. 20.16.) Ahab made a covenant with Benhadad, (1 Kings 20.34.) So we find it constantly. There is therefore no room to doubt that the word berit when use of transactions between man and man means a mutual compact. We have no right to give it any other sense when used of transactions between God and man. Repeated mention is made of the covenant of God with Abraham, as in Gen. 15.8; 17.13, and afterwards with Isaac and Jacob. Then with the Israelites at Mount Sinai. The Old Testament is founded on this idea of a covenant relation between God and the theocratic people.

The meaning of the word diatheke in the Greek Scriptures is just as certain and uniform. It is derived from the verb diatithemi to arrange , and, therefore, in ordinary Greek is used for any arrangement, or disposition. In the Scriptures it is almost uniformly used in the sense of a covenant. In the Septuagint it is the translation of berit in all the cases above referred to. It is the term always used in the New Testament to designate the covenant with Abraham, with the Israelites, and with believers. The old covenant and the new are presented in contrast. Both were covenants. If the word has this meaning when applied to the transaction with Abraham and with the Hebrews, it must have the same meaning when applied to the plan of salvation revealed in the gospel.

Secondly, that the plan of salvation is presented in the Bible under the form of a covenant is proved not only from the signification and usage of the words above mentioned, but also and more decisively from the fact that the elements of a covenant are included in this plan. There are parties, mutual promises or stipulations, and conditions. So that it is in fact a covenant, whatever it may be called. As this is the Scriptural mode of representation, it is of great importance that it should be retained in theology. Our only security for retaining the truths of the Bible, is to adhere to the Scriptures as closely as possible in our mode of presenting the doctrines therein revealed. (Online source)

Charles Hodge