I think I've come upon a devastating yet subtle 'quickie' argument against the Protestant (especially Reformed) notion of justification by faith alone. Catholics will often point out that "faith that works through love" is what Paul meant when he spoke of the essence of a justified believer (Gal 5:6), and that without love we are told by James that "faith is dead" (James 2:24-26). Protestants think they have an answer for this, by insisting (without proof) that "true faith always comes with love" with it. This seems like a save at first, but thinking about this means the Protestant is saying that when a person receives the gift of faith prior to justification, they also receive the gift of love along with it. That's a problem, and here's why.
If a person receives faith and love prior to justification, it means the unsaved individual loves God already, prior to even accepting the Gospel message! This cannot be, and thus the Protestant must reject this and say faith doesn't automatically include love along with it prior to justification. This leads to a few significant but plain conclusions:
Given the above, when Paul says we are "justified by faith," he isn't saying we are "eternally saved by faith," rather he's saying that we receive God's love within us by believing in the Gospel, and that this is just the beginning of our salvation (Rom 13:8-14; Gal 5:13-14).(1) Faith prior to justification lacks love, and thus this faith must start off 'dead'. This isn't necessarily a bad thing, just an incomplete thing, which is why justification is still needed.(2) Justification must be what bestows love, and this seems confirmed by Scripture (e.g. Romans 5:5), and thus the Protestant can no longer say justification is purely forensic, but rather infuses divine gifts into the soul.
(3) Dead faith prior to justification becomes living faith after justification by the addition of love to faith, and herein is the essence of a justified believer. This would mean it isn't Christ's Imputed Righteousness that makes all the difference, but rather the presence/absence of love, and thus suggests your justification (salvation) hinges upon what you do with that love. This is why texts like Revelation 21:8 list "unbelief" as one of the many sins that can damn a person, because it's possible to have faith and be damned by other grave sins.