In the last article about this topic, I presented a video from Dr. James White on the issue of Gen. 12 vs Gen. 15 in regards to when Abraham first believed. I think multiple ways of explaining it can occur, but here is Dr. Thomas Schreiner’s take on the passage:
A word should also be said about Gen 15:6. Beckwith points out that Abraham believed in Genesis 12 when he left his homeland and traveled to Canaan, and hence 15:6 can’t be his initial justification, for we know Abraham trusted God in Genesis 12. Hebrews 11:8 confirms that Abraham’s obedience in Genesis 12 stemmed from his faith, for we read “by faith Abraham obeyed.” Beckwith raises a fascinating issue here, but it isn’t clear that the text points to a process of justification. Genesis 15:6 doesn’t clearly teach that Abraham continued to be justified every time he exercised faith. Instead, 15:6 clarifies what is implicit in Genesis 12 and brought out by the writer of Hebrews (Heb 11:8). Genesis 15:6 explains that Abraham’s faith counts as his righteousness. Such a statement doesn’t mean that every time someone exercises faith their justification increases. Paul and the writer to the Hebrews teach that a right relationship with God is ours through faith instead of by works. Ongoing acts of faith don’t continue a process of justification; they verify the authenticity of the first act of faith.
Thomas R. Schreiner. Faith Alone—The Doctrine of Justification: What the Reformers Taught…and Why It Still Matters (The Five Solas Series) (Kindle Locations 5079-5086). Zondervan. Kindle Edition.
And we read in Gen 12: 4, “So Abram went.” Hebrews summarizes what occurred in the words, “By faith Abraham, when he was called, obeyed.” It is obvious that he obeyed, but Hebrews adds that Abraham’s obedience flowed from his faith. It is clear from what the writer tells us here that Abraham’s first act of faith did not take place in Gen 15: 6, even though Gen 15: 6 is the first time we are told that Abraham believed. He dared to leave his homeland only because he trusted in God. Such a comment does not contradict Paul’s declaration that Abraham was justified by faith in appealing to Gen 15: 6 (Rom 4: 3; Gal 3: 6). In fact, it confirms what Paul teaches, showing that faith is the root and obedience is the fruit. Abraham was promised a land (Gen 12: 1), which would be his inheritance (11: 8), but he wasn’t told the location of the land when he set out. 564 He trusted that God would reveal the place of his inheritance and that God would give it to him. Faith, the author instructs the readers, does not see the end at the beginning. Faith always trusts in the promises of God, even when it looks as if they won’t be fulfilled. The readers should do the same: they must cast themselves entirely on God and believe he will give them the final inheritance.
Schreiner, Thomas R.. Commentary on Hebrews (Biblical Theology for Christian Proclamation) (Kindle Locations 6979-6989). B&H Publishing Group. Kindle Edition.