Abraham believed in God and believed in God's word. Abraham trusted God to take care of him and to keep His promises. Abraham's trust and belief were such that God made Abraham His own, using him to fulfill His ultimate promises to mankind. Abraham was God's before the covenant was in place and before the Messiah came into the world, so his belief and trust were all he had to offer God.
We, however, have Jesus Christ. We are told that He is the way, the only way. Our faith is in Jesus Christ -- the son of God and the son of man, born of the virgin birth, crucified on the cross, died, buried, and risen after three days. This is the only way we are justified. The justification given to us by Jesus Christ bridges the gap of sin between us and God. There is no way we could ever bridge that gap alone. Jesus Christ gives us the right to approach our Father and to know that we are His.