In God's redemptive plan (who He is saving, who He is not saving) of the Old Testament dispensation, God was basically saving the Israelite nation. Israel was God's chosen people out of all the nations of the Earth and everything belonged to them (Romans 9:4-5). All the other nations were allowed to walk in their own ways (Acts 14:16), prior to Christ and the Apostolic age (Romans 10:19-20). Virtually all of the Gentiles were passed over. Rahab and Ruth are two of the only (if not the only two) Gentiles saved during that period. We have an entire age and we can name two Gentiles that were saved by God's mercy. How many of Israel were saved during that period? Only a remnant (Romans 9:27; 11:4-5). Most perished in their unbelief, as Hebrews tells us (Hebrews 3:19).
Who came into the promised land that had faith? Joshua and Caleb. The rest perished in the desert. God's redemptive plan saved only a remnant of Israel. The rest He hardened (Romans 9:14-18). God the Father speaks to God the Son in Isaiah 49:6, and when Jesus came He said many of the Gentiles will come into that kingdom (Matthew 8:11; Zechariah 8:22). Jesus' parables tell us that the kingdom will draw good fish and bad fish, wheat and tares, wise virgins and foolish virgins, genuine converts and false converts (Matthew 13:1-52; 25:1-13). Of the many who name the name of the Lord, few will enter heaven (Matthew 7:21-23). Most of the Israelites are still stumbling over Christ Jesus (Romans 11:5). God is bringing all these Gentiles in to make the Israelites jealous (Romans 10:19; 11:11).
God chooses by grace. It is not because of the will of the flesh or the will of man (Romans 9:16). God showed us Gentiles that it is by grace by putting Israel on the sidelines, hardening them to show us that it is not by Jewishness or by family lines or by any physical connection or anything man has to offer (John 1:12-13; Romans 2:25-29; 9:6-8). God breaks it down further in 1 Corinthians 1:26, showing us that there is nothing for us to boast about in regard to ourselves. It is by grace that we are saved. Faith is not our own but a gift from God (Ephesians 2:5, 8). God is going to save all of Israel (Romans 11:26)—not Israel according to the flesh, but spiritual Israel (Romans 11:17-24; Galatians 3:7, 16, 29; Ephesians 2:11-22). Many are vessels of wrath for the sake of the vessels of mercy (Romans 9:22-24; 2 Timothy 2:20). The Father has elected us from eternity, the Son has atoned for us in His death, and the Spirit irresistibly regenerates and leads us to faith.
Genesis 15:5 says, "And He took him outside and said, 'Now look toward the heavens, and count the stars, if you are able to count them.' And He said to him, 'So shall your descendants be.'"
Genesis 17:7 says, "I will establish My covenant between Me and you and your descendants after you throughout their generations for an everlasting covenant, to be God to you and to your descendants after you."
In these passages, who is the term "descendants" referring to? If you say "Israel," you are wrong.
Romans 9:6-8 says, "But it is not as though the word of God has failed. For they are not all Israel who are descended from Israel; nor are they all children because they are Abraham's descendants, but: 'Through Isaac your descendants will be named.' That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants."
After reading that passage, look again at the promises in Genesis and answer the question as to who the "descendants" are. If you still cannot answer the question correctly, read Galatians 3:7, 16 and 29.