Gospel Guidebook: Getting and Keeping It Right 
Hebrews 11:8-9 and Imputed Righteousness
Over the past few decades, New Perspective on Paul (NPP) theology has spread everywhere and Eastern Orthodoxy (EO) has also been gaining popularity in the West. In my experience, it seems to me that NPP and EO people don't have a reasonable response to the doctrine of imputation taught in Romans 4. However, in debate situations, when presented with Romans 4:3, I have noticed that some of them evade it by appealing to Hebrews 11:8-9. In particular, they point out that Abraham had faith and was walking in obedience many years before Genesis 15:6. They think this observation undermines the plain (normative) reading of justification through imputation of righteousness in Romans 4 and requires a new reading of it through the lens of covenant relationship (NPP) or transformation of life (EO). However, I don't think the plain reading of Romans 4 can be undermined so easily. In fact, it seems as if the Apostle Paul already anticipated this type of objection, and in order to preemptively deal with it, he purposefully and carefully placed the content of Romans 4:2 directly before his proof text in Romans 4:3. What good works of Abraham was Paul referring to in Romans 4:2? It must included those in Hebrews 11:8-9. In other words, Paul was excluding "obedience enhanced faith" (i.e., synergistic faith + obedience) in Romans 4:2 from his justification by faith in Romans 4:3.
John Chrysostom, who is regarded as a saint and foremost patristic of Eastern Orthodoxy, also recognized that it could not have been Abraham's synergistic "obedience enhanced faith" in Hebrews 11:8-9 or elsewhere that caused in his justification. He comments on Romans 4:1-2 as follows: "Now since the Jews kept turning over and over the fact, that the Patriarch, and friend of God, was the first to receive circumcision, he wishes to show, that it was by faith that he too was justified. And this was quite a vantage ground to insist upon. For a person who had no works, to be justified by faith, was nothing unlikely. But for a person richly adorned with good deeds, not to be made just from hence, but from faith, this is the thing to cause wonder, and to set the power of faith in a strong light." (italics mine).
John Chrysostom carefully notes that it is neither observance of the ceremonial law in circumcision nor being "richly adorned with good deeds" that could result in justification. Since "obedience enhanced faith" is a synergy of faith and obedience (i.e, good deeds), it too must be excluded. In fact, John Chrysostom in several places in his commentaries tells us it is "faith only" that justifies. For example, in his commentary on Acts 15:1, he says, "This is also meant as a lesson to those (objectors); this is able to teach even them that faith only is needed, not works nor circumcision" (italics mine). In context, these objectors were teaching that Gentiles needed to be circumcised and keep the law of Moses (Acts 15:5). But John Chrysostom reminds us that neither are required.
In light of the above, Abraham's "obedience enhanced faith" of Hebrews 11:8-9 could not have been the cause of his justification. (It might have been the result of his justification, if Abraham was justified by "faith only" at Genesis 12:1-3 or earlier, a possibility that I do grant. In particular, Moses might have been using the events of Genesis 15:1-5 as a springboard to introduce how Abraham received imputation of righteousness at a time prior to those events.)
In addition to Abraham's experience, we have another example in the New Testament of a person who was "richly adorned with good deeds" prior to being saved by faith. This occurred in the Apostle Peter's dealings with Cornelius. Peter testified of Cornelius by saying, "But in every nation he that feareth him, and worketh righteousness, is accepted with him" (Acts 10:35). Cornelius was accepted with God. God was pleased with his prayers and alms. However, as good as those things were, and regardless of whatever temporal blessings they may have procured from God, Cornelius still needed to hear the gospel and believe it to receive remission of sins (10:43) and be saved (11:14). I believe it was the same with Abraham. He believed God and was obedient before Genesis 15:6, but it wasn't until he believed the promise of God concerning his seed (i.e., Christ) that he was justified.
I don't view Hebrews 11 as dealing with justification of the sinner on the basis of faith in Christ. In contrast, I view Hebrews 11 as dealing with justification/vindication of the righteous man. The word "justification" must be understood in context. In Romans 4, we are dealing with experiential forensic justification, whereas in Hebrews 11, we are dealing with experiential ethical justification. The former is experienced by faith apart from works as an ungodly man, and the latter by faith-based works as a godly man. The latter could be viewed "vindication." (For more information on these distinctions, see my article Categories of Positional and Experiential Justification). Hebrews 11 is basically a pep talk for struggling and suffering Christians to progress in their sanctification through vindicating acts of experiential ethical justification so that they can receive rewards and inheritance (10:35, 11:6-9, 11:26) in line with the eternal life they received on the basis of their position/experiential forensic justification.
As mentioned above, I think Genesis 15:6 does provide some leeway with regard to the timing of Abraham's justification. That moment most likely happened in Genesis 15:6, granted that it follows chronologically from Genesis 15:1-5. However, it is also possible that Moses was describing a moment of belief that happened many years previously in Abraham's life, such as Genesis 12:1-3, and was simply using the events of Genesis 15:1-5 as a springboard to introduce Abraham's moment of justification. Either way, Hebrews 11:8-9 does not at all contradict the doctrine of imputed righteousness through a single moment of faith in Christ apart from works.