Belief

Gal 3:6-96 Consider Abraham: "He believed God, and it was credited to him as righteousness." 7 Understand, then, that those who believe are children of Abraham. 8 The Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: "All nations will be blessed through you." 9 So those who have faith are blessed along with Abraham, the man of faith.
NIV

Abraham was not perfect, none of us are, but he believed God. He believed enough to listen to God and obey what he heard. He placed his full trust in God, and in doing so was judged as righteous. Did he have doubts along the way? Did he wonder how in the world or when things would be accomplished? You know he did, but still his trust remained in the Lord. If he had been in the Garden of Eden perhaps that story might have ended differently.

Because of that faith, God revealed to this man how He would lift up this fallen world. All those who place their faith in Him would be blessed, just as Abraham was blessed.

We become children of Abraham if we believe as he did, and thus we become children of God. Lack of belief caused the world to fall. How fitting it is for belief to be the only way back to God.