Berith is the Hebrew word for covenant — a solemn, binding agreement — and it is one of the keys to the whole Bible. God relates to his people by covenant: with Noah, sealed by the rainbow; with Abraham; with Israel at Sinai; with David. A covenant is more than a contract; it is a bond of committed relationship, sworn and sealed.
The prophets looked forward to a berith chadashah — a new covenant, in which God would write his law on human hearts and remember sin no more. Jesus announced its arrival at the Last Supper: this cup is the new covenant in my blood. The whole Bible divides into old covenant and new — and the new is sealed not with the blood of animals, but of Christ.
Berith in Scripture
But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.