"And he [Abram] believed in the Lord; and he counted it to him for righteousness." -Genesis 15:6, KJV

Salvation works the same way today that it did 4000 years ago. Abram (Abraham) got saved the same way we do: maybe not by saying a neat little prayer, but through faith. When he was given a promise from God, he simply believed, and that belief is what made Abram righteous in God's eyes. Salvation has always been through faith, not by works (even though faith usually does shows itself through works--if a person has faith, he can be used of God). God didn't change the plan when Christ came, He clarified and fulfilled it. God never changes, and neither has His plan. God is constant, giving us Someone to rely on in this unstable world.

"Jesus Christ the same yesterday, and to day, and for ever." -Hebrews 13:8, KJV