This comes to mind in regard to a recent post. In it we see St. Thomas quoting Luke 7:47: "Many sins are forgiven her, because she hath loved much." The observation to be made here is that Christ attributes the forgiveness of sins to Mary Magdalene's love, not to her faith. But this flies in the face of the Protestant's claims about sola fide, wherein he says that he is saved by faith in Christ alone.
Now the Catholic does not have difficulties with this passage: we acknowledge what it says, and we say with St. Thomas (as discussed in the aforementioned post) that God rewards what he has given. But it seems to me that the Protestant cannot easily "handle" this declaration of the Lord within his own system.
Of course, this is not the only passage that undermines sola fide. When Matthew and Mark report the Lord's preaching, it is this: "Repent, for the kingdom of heaven is at hand" (Mt. 4:17) and "Repent and believe the gospel" (Mk 1:15). In the first case, there is no mention of faith; in the second, it is inseparable from faith. Faith and action go together.
Just as telling is St. Peter's first sermon, following which the crowds say, "Men and brethren, what shall we do?" St. Peter does not take the road of sola fide here. He does not tell them, "You do not have to do anything. Just have faith in Christ." No. He does nothing of the sort. He tells them what they must do: "Repent, and be baptized..." (Acts 2:38). Now we Catholics certainly agree that one must believe in order to be saved; mere deeds are not enough. We are not legalists. So clearly an unstated premise of what St. Peter says is that one must believe.
But the Protestant who believes in salvation by faith alone cannot deal with St. Peter's declaration on his own sola fide terms. St. Peter calls for action; the Protestant calls for faith alone. Did St. Peter get it wrong? Of course not! But that means that the call for salvation by faith alone is what is wrong, because it simply cannot be reconciled with St. Peter's call for action in order to be saved.