Going by vague memory (I simply don’t have time right now to do anything detailed) Turretin answers the question about why faith is singled out in justification and salvation by saying that it, unlike the virtues of hope and love, is outward looking.

I just don’t see it.  Hope seems like it could be expressed in almost identical terms.  In fact, the Apostle Paul sometimes uses the term, “set one’s hope on God” in a way that seems identical to “trust in God.”

And love too, it seems to me, could be understood not as a form of working for wages, but rather an empty hand that receives all from the beloved.

I’m not sure that psychological analysis of the nature of the virtues involved is a profitable way to figure out why faith alone justifies (which it does, by the way).

Looking at Hebrews and Romans and elsewhere, I wonder if the answer shouldn’t be sought in God’s and Christ’s virtues, rather than in the nature of faith.  Maybe faith answers to God’s faithfulness, and the point is that our only hope lies in God’s promises and his character which demands that he fulfill those promises.

We trust God because he is trustworthy, and the Gospel reveals his trustworthiness.  For in the Gospel, God’s righteousness is revealed from His trustworthiness to our trust.

“By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised” (Hebrews 11.11).