Most readers with even a passing interest in apologetics are familiar with St Paul's words from Romans 4:3-5,
For what does the Scripture say? "Abraham believed God, and it was counted to him as righteousness." Now to the one who works, his wages are not counted as a gift but as his due. And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousnessThis passage is critical to the doctrine of Sola Fide because it is where Protestants claim St Paul expressly lays down the doctrine of the 'Imputation of Christ's Righteousness' to the believer at the (one and only) moment of Justification, and that this Righteousness is received by faith. The standard and historic Protestant interpretation of "faith is counted as righteousness" is clearly stated in the Westminster Confession (XI:1):
Those whom God effectually calls, He also freely justifies; not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for any thing wrought in them, or done by them, but for Christ's sake alone; nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on Him and His righteousness by faith; which faith they have not of themselves, it is the gift of God.What this passage is saying is that the phrase "faith counted as righteousness" is not to be understood as 'faith itself is counted as righteousness,' but rather, 'faith receives Christ's Righteousness.' What this article will demonstrate is effectively a silver bullet right to the heart of this heresy.
Now, to my knowledge, nobody has made the argument I'm about to make. But before I do so, I will first lay out (in no particular order) some classical (and very solid) Catholic arguments against the Protestant interpretation:
- Abraham was not converting in Genesis 15:6, and in fact walked with God years prior to this moment in his life (e.g. Gen 12:1-4, c.f. Heb 11:8). Thus, he couldn't have been undergoing what Protestants consider "justification." Now, they might object that Paul wasn't concerned about timing (even though he clearly was, 4:9f, 18ff), but that doesn't help their bind for Gen 15:6 was a historical event and something relating to salvation clearly took place.
- Psalm 106:30f states Phinehas' good work was "counted as righteousness" (not his faith, though he certainly was already a believer). This phrase is identical in Hebrew and Greek to what is said in Gen 15:6. The Protestant cannot have this, for good works don't receive Christ's Righteousness, thus they seek to interpret this verse differently. The problem here is that one should generally assume identical phrases are interpreted the same (unless a reasonable case can be shown otherwise), and Catholics do have such an interpretation harmonizing the two - but the Protestant misses this point because they are busy trying to salvage Sola Fide.
- The text plainly says "faith" is what is reckoned as righteousness, and Romans 4:18-22 is devoted to unpacking Genesis 15:6, stating: "he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised. This is why "it was credited to him as righteousness."" This likewise fits how the Bible defines faith (e.g. Heb 11:1,6), and thus not some notion that faith is of no inherent value but merely acts as an instrument which transfers an alien righteousness.
- When Scriptures (especially the NT) uses the Greek/Hebrew term "counted" it quite frequently means "consider something as it really is," while rarely meaning "consider something to be other than what it really is."  (In Romans 4 this would indicate an internal/infused righteousness, rather than an external/alien righteousness.) Thus the Protestant must approach Romans 4 with a bias, and without establishing a sufficient case for their decision, they're begging the question.
With this in mind, we now note that this verse is located in a critical context: sandwiched between "counted as righteousness" in verse 3 and verse 5! Barring any desperate attempt to say Paul shifted the meaning of "counted" from verse 3 to 4 and then back again in verse 5, suggesting Paul engaged in equivocation, the Protestant position is indefensible.
What is most astonishing about all these points is that Catholics are simply letting Scripture speak (which fully support the Catholic position), while the Protestant position (ironically, from a Sola Scriptura point of view) has to read into the text all sorts of preconceived notions and dance around the glaring difficulties.
 See my article: Full Biblical analysis of "counted."