Abraham

God made a covenant with Abraham that his son with Sarah would be their heir.  And he believed the Lord, and he counted it to him as righteousness.(Genesis 15). However, because Abraham was 86 years old and Sarah was 76 years old and without children, Sarah did not believe, so she gave her Egyptian servant, Hagar, to Abraham to have a child with. Hagar had a son name Ishmael. However, the promise did not go through Ishmael because the covenant had already been made. God did see that Hagar and Ishmael would be greatly blessed (Genesis 16).

The Arab nations of today are the descendants of Ishmael, so both the Jews and Arabs are descendants of Abraham. That has been cited as why they struggle so much in general, and especially over the "Tomb of The Patriarchs" in Hebron, in which Abraham is buried, right to this very day.

"Then God said, "Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish My Covenant with him as an everlasting covenant for his descendants after him." (Genesis 17:19).

They did have their son, Isaac, when Abraham was 100 years old and Sarah was 90 (Genesis 21:5).  When Isaac was a kid, God tested Abraham by commanding him to "Take your son, your only son Isaac, whom you love, and go to the region of Moriah. Sacrifice him there as a burnt offering on one of the mountains I will tell you about." (Genesis 22:2). Abraham passed the test by following God's command, but God stopped it just in time, "Do not do anything to him. Now I know that you fear God, because you have not withheld from Me your son, your only son." (Genesis 22:12). Abraham obeyed because he thought that God would raise Isaac from the dead to keep His promise (Hebrews 11:17-19).

Abraham had been willing to sacrifice his only son to God, just as God Himself would sacrifice His only Son. An interesting note is that Mount Moriah, is today known as the Temple Mount in Jerusalem. Therefore, Isaac would have been sacrificed very near the place that Jesus Christ was Crucified. This was a foreshadowing and a type of Jesus. After Abraham died, he was buried in the "Tomb of The Patriarchs" which is the burial place of Abraham, Sarah, Isaac, Rebecca, Jacob/Israel and Leah. (Genesis 35:28-29, 49:31).

Abraham is mentioned the most of any old testament figure in the new testament. He is mentioned as the original Jewish patriarch, the father of the chosen people of God, in Matthew, Mark, Luke, John, Acts, Romans, 2 Corinthians, Galatians, Hebrews, James, and 1 Peter. He also mentioned as have been justified by his faith, not works in Romans, Galatians, Hebrews, James.