After Abraham was willing to sacrifice his beloved son, Isaac (a type of God, the Father, who sacrificed His only Son, Jesus, on the Cross), God promised him: 'I will surely bless you and make your descendants as numerous as the sand on the seashore' (Genesis 22:17). If we look at the country of Israel, where many of Abraham, Isaac and Jacob's descendants live, we have to use a lot of imagination to think they will eventually be as numerous as the sand on the seashore. However, if we look at God's promise to Abraham a bit more carefully, we notice that there is more to it: '...and all the peoples of the earth will be blessed through you' (Genesis 12:3b). That was a reference to Jesus who would be one of the offspring of Abraham, Isaac and Jacob (see the genealogy of Jesus in Luke 3:23-38).
Jesus changed history. He also changed the histories of individuals and their genealogies. It is interesting that in the Old Testament the people are described by their genealogies (their bloodline): 'the son of..., the son of...' (see 1 Chronicles 1-8 as well as Luke 3:23-38). In the New Testament the genealogical lists are much rarer than in the Old Testament. Why? It has to do with changed genealogies. No longer are people bound to their natural genealogies, like the son of Mister and Missus So and So, but they can become children of God. They become offspring of Abraham: 'Understand then that those who believe are children of Abraham' (Galatians 3:7). Note the emphasis on 'those who believe.' They are those who believe God's Word and have been saved by the blood of Jesus and filled with the Holy Spirit. We find the same genealogical possibility change in the book of Romans: 'It is not the natural children who are God's children, but it is the children of the promise who are regarded as Abraham's offspring' (Romans 9:8). This is confirmed by the parable of the rich man and Lazarus which was told by Jesus (Luke 16:19-31). When he was in hell, in torment, the rich man saw Abraham with Lazarus by his side (verse 23). The King James Version of the Bible translates it as: '...and [he] saw Abraham afar off, and Lazarus in his bosom.' Abraham had embraced another one of his children that had died in this physical world and moved into the presence of Abraham.
Conclusion: God promised Abraham offspring as numerous as the sand on the seashore (we cannot say how many that is, but we do know that if it were possible to count the saved, it would be many billions and they are all Abraham's descendants). God never lies. He is still keeping His promise to Abraham. It was made thousands of years ago in the Old Testament and is explained more fully in the New Testament. That shows that the Old and New Testaments are both essential parts of God's revelation to man. One cannot neglect one and favour the other. Both are necessary to give the big picture of God's plan for man, the establishing of His eternal Kingdom on earth.