“And he [Abraham] believed in the LORD, and He [God] accounted it to him for righteousness.”
Genesis 15:6

God promised to make of Abraham a great nation. But alas, Abraham had no children. Abraham believed God, but even Abraham’s faith was tested when, in Genesis 15, Abraham was 75 years old and still childless. Abraham questioned God (not disrespectfully) “Where is the promise?” (v.2-3). God restated His promise (v.4-5), with a formal covenant that symbolized that God would fulfill His promise irrespective of what Abraham might do or not do (v.9-17).

In the midst of this narrative, even in spite of Abraham’s wobbly faith, God counted Abraham’s faith to him for righteousness.

From Abraham came a family that eventually became the nation of Israel. How has Israel blessed all the families of the earth? Besides being the nation through whom God gave His Law and sent His prophets (Romans 9:4), it was through the nation of Israel that God sent Jesus, the Messiah, the Savior (Romans 9:5).

In case there is any question regarding Jesus being the fulfillment of God’s promise, Galatians 3:16 makes it abundantly clear:

Now to Abraham and his Seed were the promises made. He does not say, "And to seeds," as of many, but as of one, "AND TO YOUR SEED," who is Christ.

Genesis 15:6 is the first clear teaching in the Bible that God calls people righteous, not because of their ethnicity or because of what they do, but because they believe (or trust) GOD. (Paul points this out in Romans 4:1-3).

What is the promise one must believe to be declared righteous? The promise of Jesus the Messiah—given to Abraham prophetically, and fulfilled in Jesus, the Christ.

The story of Jesus did not begin with His birth in Bethlehem in the New Testament, but in Genesis, the first book of the Old Testament.