Verse 6. Even as Abraham believed God, etc. See this passage fully explained See Barnes "Ro 4:3".

The passage is introduced here by the apostle to show that the most eminent of the patriarchs was not saved by the deeds of the law. He was saved by faith, and this fact showed that it was possible to be saved in that way, and that it was the design of God to save men in this manner. Abraham believed God, and was justified, before the law of Moses was given. It could not, therefore, be pretended that the law was necessary to justification; for if it had been, Abraham could not have been saved. But if not necessary in his case, it was in no other; and this instance demonstrated that the false teachers among the Galatians were wrong even according to the Old Testament.

{b} "Abraham believed" Ge 15:6 {1} "was accounted" "imputed" {*} "righteous" "in order to"