Throughout Genesis, the God of promise is portrayed as stronger than the obstacles that arise through the unfaithful actions of His people. In Genesis 18-21, the primary obstacle to God’s promise is the sin of unbelief. When the three heavenly visitors came to Abraham in Genesis 18 to affirm the Lord’s promise that Abraham and Sarah would conceive—despite their advanced years—Sarah responded with sarcastic laughter (Gen 18:12). But God was not joking. The Lord’s response set forth the theme of salvation for all who look beyond their total inability to a God who is faithful to His word. The Lord asked Abraham, “Is anything impossible for the LORD?” (Gen 18:14; see Luke 1:37).
Finally, after 25 years of waiting, Abraham and Sarah were biological parents of a son, confirming God’s word in Gen 12:1-3 and 15:1-6. Sarah’s response represents the joy of all who experience God’s faithfulness despite seasons of unbelief: “God has made me laugh,” she exclaimed, “and everyone who hears will laugh with me” (Gen 21:6). But the arrival of the child of promise sparked tension in Abraham’s family. Sarah recognized that Ishmael, the son born of natural means through Hagar, was a threat to Isaac (Gen 21:8-21). The work of God and the work of man would have to be separated.
God’s faithfulness to His words of judgement and promise in Gen 18-21 established a framework for New Testament authors to help their audiences understand God’s acts in the new covenant.
(1) In Galatians 4 Paul employed episodes from the life of Abraham in Genesis 16-21 to help the churches of Galatia understand Christian freedom from the Mosaic law. Paul argued that Abraham’s first two children were representative of the old and new covenants. Hagar, the mother of Ishmael, conceived through the natural laws of procreation (Genesis 16). Hagar corresponded to Mount Sinai and the Mosaic law (Gal 4:24-25). But Sarah, who conceived in her old age, received her child by promise and faith (Genesis 21). In this way, Paul argued, while Ishmael was the child of slavery to the law, Isaac was the child of freedom and promise (Gal 4:26-30). Isaac’s birth was the work of God and not man. Paul proposed that believers—born by the supernatural work of the Spirit—are children of promise, corresponding to the child of promise, Isaac (see Rom 9:7), while those desiring to be under the Mosaic law are slaves, corresponding to Hagar’s child Ishmael. Abraham’s fleshly act with Hagar represented a lack of faithful living. By submitting to the Mosaic law and circumcision, the Galatians were in danger of succumbing to a life of slavery. Those attempting to display their faith in Christ through the Mosaic law had no hope of eternal inheritance. Paul admonished them quoting from Gen 21:10, “Throw out the slave and her son, for the son of the slave will never inherit with the son of the free woman” (Gal 4:29). For Paul, religious practice was an expression of identity—and he reminded the Galatians: “Brothers, we are not children of the slave but of the free woman” (Gal 4:31).
(2) Peter and Jude noted that just as God was faithful to His word of judgement against Sodom and Gomorrah, He would be to His word of judgement against those who opposed the message of Jesus. In 2 Pet 2:5-7, Peter wrote that though the Lord saved Lot from destruction, He destroyed the residents of Sodom and Gomorrah. Peter wanted his readers to understand that the Lord would destroy in like manner those who blasphemed God and prophesied lies concerning the day of Jesus’ return. In Jude 7, Jude cited God’s judgement of Sodom and Gomorrah as an example of how God treats those who persist in sin and deny the Master, Jesus Christ.