Genesis 22:18

And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

Because thou hast obeyed my voice - i:e, executed my command. Abraham's acceptance with God is represented in the New Testament sometimes as the consequence of his faith, at other times as the reward of his obedience. There is no discrepancy in these statements. Abraham received the promises from God out of His free grace, and unconditionally. He believed them, and his faith was counted to him for righteousness, whilst he was yet an uncircumcised man. His faith was manifested and proved by his unreserved obedience (James 2:21), and received the highest tokens of divine approval. 'God first promises, and by his revelation awakens faith in the heart of Abraham; he then crowns with reward the works of this faith which is the result of His grace' (Gerlach).

This, then, is the Abrahamic covenant, which presents a two-fold aspect-the one fleshly (namely, the Hebrew nation), and the other spiritual. The grand features of this covenant are, first, its culminating point, Christ; secondly, its universality, embracing "all the nations of the earth;" and, thirdly, it's immutability, being confirmed by a peculiarly solemn oath, which was never repeated to the patriarchs, although frequent allusions were made to it (Genesis 24:7; Genesis 26:3-4; Genesis 26:24; Exodus 13:5; Exodus 13:11; Exodus 33:1; Hebrews 6:13-14). This sacrifice brought Abraham into a new relation to his posterity, because it formed the inauguration of the dispensation of grace (Galatians 3:8). The promised advent of Christ is inseparably associated by God himself with this great act of faith; and Christianity is the full development of the Abrahamic covenant, because all believers are the seed of Abraham.