Romans 4:18-22
18 “In hope against hope he believed, so that he might become a father of many nations according to that which had been spoken, “So shall your descendants be.” 19 Without becoming weak in faith he contemplated his own body, now as good as dead since he was about a hundred years old, and the deadness of Sarah’s womb; 20 yet, with respect to the promise of God, he did not waver in unbelief but grew strong in faith, giving glory to God, 21 and being fully assured that what God had promised, He was able also to perform.22 Therefore it was also credited to him as righteousness.”
Because righteousness is a free gift, even though Abraham did waver by sleeping with Hagar, God still saw him as having never wavered. This is why Abraham was not weak in the faith: His righteousness is based not on works, but belief. Christians often have trouble accepting this truth for themselves. We think that if we waiver or are faithless, that it somehow nullifies God’s promise. But Paul’s whole point is that this is not possible because of how God has rigged in the system. We are saved through faith, apart from the law, so that our salvation is never nullified.
