And he believed in the LORD; and he counted it to him for righteousness.

Wesley's Notes

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.

Matthew Henry's Concise Commentary

15:2-6 Though we must never complain of God, yet we have leave to complain to him; and to state all our grievances. It is ease to a burdened spirit, to open its case to a faithful and compassionate friend. Abram's complaint is, that he had no child; that he was never likely to have any; that the want of a son was so great a trouble to him, that it took away all his comfort. If we suppose that Abram looked no further than outward comfort, this complaint was to be blamed. But if we suppose that Abram herein had reference to the promised Seed, his desire was very commendable. Till we have evidence of our interest in Christ, we should not rest satisfied; what will all avail me, if I go Christless? If we continue instant in prayer, yet pray with humble submission to the Divine will, we shall not seek in vain. God gave Abram an express promise of a son. Christians may believe in God with respect to the common concerns of this life; but the faith by which they are justified, always has respect to the person and work of Christ. Abram believed in God as promising Christ; they believe in him as having raised him from the dead, Ro 4:24. Through faith in his blood they obtain forgiveness of sins.