I’m sorry that I am not familiar with the alleged Protestant theologian who maintains this teaching. Can you cite a source, so I may verify it for myself? Such a position is not consistent with what I know of Reformed theology. But I could be wrong, maybe you can provide concrete evidence that I am, rather than merely assert it without documented support.
I would agree that Abraham would have to have faith prior to Gen 12, because that faith motivated the obedience to follow God’s direction. And he couldn’t have that faith unless God had given it to him by grace, which in turn means that he was justified by that faith, once and for all time.
Why is this so. Because Romans 8:8 makes it clear that a sinful man cannot please God. Therefore, in order to obey God, thereby pleasing God, Abraham’s sin must have been pardoned, and Abraham thereby justified before God.
The only alternative would be to say that Abraham obeyed God, before he was actually given faith (which justifies the sinner). This would be a Pelagian error. Would it not?