In Genesis 15 God speaks to Abram several times:
"Do not be afraid, Abram.
I am your shield,
your very great reward.
"...a son coming from your own body will be your heir... Look up at the heavens and count the stars -- if indeed you can count them. So shall your offspring be.
"I am the Lord, Who brought you out of Ur of the Chaldeans to give you this land to take possession of it.
"Know for certain that your descendants will be strangers in a country not their own, and they will be enslaved and mistreated four hundred years. But I will punish the nation they serve as slaves, and afterward they will come out with great possessions. You, however, will go to your fathers in peace and be buried at a good old age. In the fourth generation, your descendants will come back here, for the sin of the Amorites has not yet reached its full measure.
"To your descendants I give this land, from the river of Egypt to the great river, the Euphrates -- the land of the Kenites, Kenizzites, Kadmonites, Hittites, Perizites, Rephaites, Amorites, Canaanites, Girgashites and Jebusites."
This kind of reiterates what God told Abram in chapter 12 ("I will make you into a great nation and I will bless you; I will make your name great, and you will be a blessing..."), but it specifies that this great nation will begin with his very own son. It says that Abram believed the Lord, and God "credited it (believing) to him as righteousness. This is like a foreshadowing of us believing the Lord and being credited with the righteousness of Jesus, Who lived a sinless life and became a sacrifice for us.
It's like God looked at His Son while on the cross and saw us; and He looks at us right now and sees His Son. We've been imparted with an incredible reputation and been "credited" with righteousness. This is no small thing.