"I speak after the manner of men" points to the fact that men enter into agreement, not with a heavenly language, for that would be unlawful for man to speak (2 Cor. 12:4), and a man has not the capacity to comprehend that which is spoken on a heavenly level. The word must be plain, concise, and complete so that each participant can with complete understanding enter into an agreement. Neither can anyone outside those parties break the agreement or change it in any way. This is an agreement between two men only, and even the law itself cannot interfere with its content or purpose.
Even though the covenant that God makes with man is in human speech, the fact that it is in man's words does not make the covenant void if it is confirmed. Heb 6:18 That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:
A covenant from God is confirmed by two immutable things: God himself and the oath that he has made. God cannot change; God cannot lie. Therefore, any covenant that God makes with man is one that cannot be interrupted or changed by man's doing. It is a firm foundation upon which we can believe and depend. No man can disannul it nor add to it. This is the reason that we cannot add to nor take from the Gospel message concerning Jesus Christ and his righteousness. This is the reason that we cannot add to nor take away from the very words of God, which are written in the Bible. The Gospel and the Word of God are a covenant made with man and confirmed by God and his oath.
The promises that were "made" (REO, meaning literally "to flow out" or "to speak") were not here first made as in beginning the promises. The promises of God to his people were made in timeless eternity past and through God's Son, Jesus Christ. Here the promises are reiterated (re-affirmed) through Abraham and his seed (See Genesis 17:7).
These promises were not to many seeds, many Gospels for many people, especially speaking of the Jews and Gentiles, no Gospel for Gentiles with another for Jews, etc. There is only one seed (Christ), and the promises are centered in him alone. The promises were given only to Abraham's seed, and that spiritual seed, since all who believe are the children of Abraham (Gal. 3:7; Rom. 2:29; Gal. 3:29).
One Abraham, one covenant, one Gospel, one Christ, and one salvation. "No man cometh unto the Father, but by me" (John 14:6).