To get at the nature of that faith, it is helpful to ponder why faith alone justifies. Why not love, or some other virtuous disposition? Here’s the way J. Gresham Machen answers this question in his 1925 book, What Is Faith?
The true reason why faith is given such an exclusive place by the New Testament, so far as the attainment of salvation is concerned, over against love and over against everything else in man… is that faith means receiving something, not doing something or even being something. To say, therefore, that our faith saves us means that we do not save ourselves even in slightest measure, but that God saves us.
In other words, we are justified by faith alone, and not by love, because God intends to make it crystal clear that he does the decisive saving outside of us, and that the person and work of Christ are the sole ground of our acceptance with God.
A hundred years earlier Andrew Fuller (the main rope holder in England for missionary William Carey in India) gave the same explanation:
Thus it is justification is ascribed to faith, because it is by faith that we receive Christ; and thus it is by faith only, and not by any other grace. Faith is peculiarly a receiving grace which none other is. Were we said to be justified by repentance, by love, or by any other grace, it would convey to us the idea of something good in us being the consideration on which the blessing was bestowed; but justification by faith conveys no such idea.
– John Piper, Think at 70
Isn’t the gospel just astoundingly beautiful?