I, the LORD, have called you in righteousness; I will take hold of your hand. I will keep you and will make you to be a covenant for the people and a light for the Gentiles. (Isaiah 42:6)And again
I will keep you and will make you to be a covenant for the people (Isaiah 49:8)When the covenant with Israel was made at the time of the exodus from Egypt, Moses then took the blood, sprinkled it on the people and said, "This is the blood of the covenant that the LORD has made with you" (Exodus 24:8)
It is an odd thing to our modern minds, but in the books of Moses, sprinkling with blood was a traditional sign of being cleansed, being sanctified, and in the case of the whole people being sprinkled, a sign of the covenant with the LORD. This ties back to Isaiah's prophecy of the suffering servant, "so shall he sprinkle many nations" (Isaiah 52:15). The covenant is now for all nations.
Jesus made known the new covenant the night he was betrayed, saying, "This is my blood of the covenant, which is poured out for many for the forgiveness of sins." (Matthew 26:28). The covenant is a binding agreement, where God has promised with blood that he will keep his word. His word to which he has bound himself is this: our forgiveness.
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