After Melchisedec as a type of Christ as God’s High Priest, we have Sarah and Hagar as types of the two covenants which God made with His chosen people concerning their inheriting of the good land. These are signified by Abraham’s wife and his concubine.
The two women, Abraham’s wife Sarah and his concubine Hagar, recorded in Genesis 21, symbolize two covenants (Gal. 4:24). One is the covenant of promise given to Abraham, which is related to the new testament, the covenant of grace; the other is the covenant of law given to Moses, which has nothing to do with the new testament. Sarah, the free woman, symbolizes the covenant of promise, and Hagar, the maidservant, the covenant of law.
Genesis 17:15-19 shows us that God definitely promised that Abraham would have a son, Isaac, and that this son would be born of his wife, Sarah. God’s promise of a seed to Abraham was altogether worked out by God’s grace. In Genesis 18:14 God said to Abraham, “At the appointed time I will return to you, according to the time of life, and Sarah shall have a son.” This means that Isaac’s birth was a birth at the appointed time, at the time of life, according to God’s promise. It was not by the natural energy of Abraham and Sarah; it was the issue of the visitation of God’s grace. God’s promise is fulfilled by God’s grace for those who inherit the promise. This grace is the very Christ Himself who brought with Him God’s grace (John 1:14). He supplies God’s chosen people with all that He is to fulfill for them God’s promise.
God’s promise is not fulfilled by the energy of man’s flesh. When God promised that Isaac would be born of Sarah, Abraham, being old and well stricken in age, considered his body as already dead, and Sarah’s womb also was deadened (Gen. 18:11-14). It was only after they became nothing and were void of strength that God came in to carry out His promise by His grace. This shows us that it is God who promises and it is also God who fulfills what He has promised. Man’s flesh is of no use at all. Abraham brought forth a seed, Ishmael, by the endeavor of the human flesh, but Ishmael was rejected by God (17:18; 21:10) and could not fulfill God’s purpose. Likewise, whatever we do, whether good or bad, by the strength and ability of our natural man cannot fulfill God’s promise to us. God desires that we only receive His grace and cooperate with Him and not do anything by ourselves.
In Genesis 21:12 God said to Abraham, “For in Isaac shall your seed be called.” Isaac was born of the free woman, Sarah, through promise. This means that he was born through God’s power in grace, which is implied in God’s promise (Gal. 4:23); that is, he was born according to God’s Spirit of life (v. 29). Only the children of promise were accounted as Abraham’s seed (Rom. 9:8), and only they could inherit the good land that God had promised to Abraham for the accomplishment of His purpose. The good land, which is a type of the all-inclusive Christ, was given by God as an inheritance to His children of promise, in which and by which they might live to defeat God’s enemy, to gain the kingdom for God, to build God’s dwelling place, and to express God’s glory.