Now let us see why this biblical proof is flawed. What does the biblical record show? My Bible tells me that Abraham received his call from the Lord when he was seventy-five years old. He did believe the Lord, and he left his home country on the basis of this faith [Gen. 12]. Did the Lord declare him righteous the moment he believed? Perhaps he did, but there is no such record in the Bible.
Then a famine forced Abraham to go to Egypt. Chapters 13 record the story of how he and his nephew lot separated due to the increase in their cattle. All this takes time, of course. Chapter 14 relates how Abraham rescued Lot. Then comes chapter 15 where God declared Abraham righteous. My estimate is that Abraham must have been by now anywhere between 80 and 85 years old or perhaps even older.
My question is: Why did the Lord wait all these years before publicly declaring Abraham righteous? We don't know. Perhaps he didn't want to issue such a declaration on the basis of incipient, untested faith. Nevertheless, regardless of the reason, the fact remains that we have no biblical proof that Abraham was declared righteous the moment he believed! Someone once said: "True faith is never alone!"