“I will answer...I will help.” The Hebrew is written in the past tense and is the prophetic perfect idiom, referring to something in the future as if it was in the past to show that the event will happen. In this context there are several indications that the prophetic perfect idiom is being used. One is that the subject of the sentence, who is the Messiah, has not even been born yet, so to say that God has answered and has helped him has to be idiomatic. Another is that part of the sentence “and I will preserve you” is in the imperfect tense, here referring to the future. This is a verse where translating the Hebrew literally as a past tense action only confuses the reader because English does not have a clear prophetic perfect idiom. [For more on the prophetic perfect idiom, see commentary on Ephesians 2:6.
“as a covenant.” The phrase “as a covenant” (or, “for a covenant”) is general and thus has a number of meanings relating to the death of Christ and the covenants. Just as a covenant binds parties together, so also Jesus Christ would bind people to God. Also, the fact that Jesus Christ was given “as a covenant” indicates that in him the Old Covenant was fulfilled and the New Covenant ratified. Jesus was both the sacrifice that was required because the Old Covenant was broken (as per Matt. 26:28) and the blood sacrifice that ratified the New Covenant (as per Luke 22:20; 1 Cor. 11:25. Cp. Exod. 24:8). Thus the word “covenant” is a metonymy for a “covenant sacrifice;” both the sacrifice that had to be offered when a covenant was broken, and the blood sacrifice that ratified and began a covenant.