Paul states in Romans 4:22 that there was a reason that righteousness was imputed to Abraham ('therefore'). The reason (or reasons) are stated in 4:16-21:
- He believed God (who quickens the dead and calls those things that be not as though they were)
- Against hope he believed in hope (concerning the promise he would father many nations)
- He was not weak in faith considering his own body and Sarah's
- He staggered not in unbelief but was strong in faith giving glory to God
- He was persuaded that God was able to perform what He promised
My problem is this appears to contradict what Paul said earlier in the chapter. In Romans 4:3 Paul had just quoted Genesis 15:6 to show that righteousness was imputed to Abraham before he was circumcised. I notice that Genesis 15 is prior to the birth of Ishmael and at least 14 years prior to God's promise of Isaac in Genesis 17, at which time Abraham was 99. Why then does Paul discuss Abraham's faith in Genesis 17 and use it as the reason God had previously imputed righteousness to Abraham in Genesis 15? That Paul does so is clear, at minimum, because of point (3) above. Abraham's body was not said to be dead in Genesis 15. In fact, it appears that neither Abraham nor Sarah thought Abraham's body was "dead" at that time because Sarah later gave Hagar to Abraham in order to father a son.
Correctly understanding Abraham's imputed righteousness seems very fundamental. I'm not sure I understand what Paul is doing here. The best I can come up with is that the nature of justifying faith is directly related to faith in God's power to raise the dead (see also Romans 4:24-25 and 10:8-10) and that Abraham had such faith in Genesis 15 though it was not proved in time until Genesis 17 (and later in Genesis 22).
Paul clearly knew the Scriptures. Why did he intentionally conflate Genesis 15 and 17 when explaining why God imputed righteousness to Abraham?