John Wesley on Genesis 15:6

And he believed in the Lord - That is, believed the truth of that promise which God had now made him, resting upon the power, and faithfulness of him that made it: see how the apostle magnifies this faith of Abram, and makes it a standing example, Rom 4:19 - 21. He was not weak in faith; he staggered not at the promise: he was strong in faith; he was fully persuaded. The Lord work such a faith in every one of us. And he counted it to him for righteousness - That is, upon the score of this he was accepted of God, and, by faith he obtained witness that he was righteous, Heb 11:4. This is urged in the New Testament to prove, that we are justified by faith without the works of the law, Rom 4:3,Gal 3:6, for Abram was so justified, while he was yet uncircumcised. If Abram, that was so rich in good works, was not justified by them, but by his faith, much less can we. This faith, which was imputed to Abram for righteousness, had newly struggled with unbelief, Ge 15:2, and coming off, conqueror, it was thus crowned, thus honoured.