Genesis 15:6 - Matthew Poole's English Annotations on the Holy Bible

He believed in the Lord, i.e. he was fully persuaded that God was able to fulfil, and would certainly fulfil, the promise made to him concerning a child, and especially concerning the Messias, who should come out of his loins by that child, and that both himself and all people should be justified and blessed in and through him. He counted it to him, or reckoned, or imputed, as this word is translated, Romans 4:10,22, for righteousness, i.e. for a righteous and worthy action, as Psalms 106:31; and further, in respect of this action and grace of faith, whereby he relied upon God for the promised Seed, and upon the promised Seed too, he pronounced him a just and righteous person notwithstanding his failings, which even this history acquaints us with, and graciously accepted him as such; which sense is easily gathered from St. Paul's explication and accommodation of this passage, Romans 4:9,18,22.