A while back Matt Jenson wrote a brilliant short essay entitled: Faith is nothing at all. Do read it if you haven't already, it won't take long.
We must constantly remind ourselves that faith is not a thing. It is not a possession by which we make claim to salvation. Faith is the absence of a thing - it is the confession of a complete lack. To even ask 'Am I having faith?' is already an unbelieving question for faith is looking away to Christ.
If you make faith into a thing you run into problems. Either you have to make it an imputed substance which God grants arbitrarily (in order to uphold sovereign grace). Or you make it a legitimate factor contributing to our salvation. Sounds quite like many Calvinist-Arminian debates right? In many (certainly not all, but in many) of these debates you can see both sides making this mistake: they begin by considering faith to be a thing. And from this premise, one side is in danger of making salvation a matter of divine caprice unrelated to Christ. The other side begins from the same premise and makes salvation a matter of self-effort (and again Christ's position is diminished). But both have begun down the wrong track. They've thought of faith as a thing and then they've got into trouble figuring out how a gracious salvation can be 'by' this thing. We must remember though: Faith is not a thing.
Alan Torrance is fond of pointing out that reformers like John Knox spoke very little about 'salvation by faith alone.' Instead he spoke of salvation 'by the blood of Christ alone.' Why? Because he didn't want anyone thinking that faith was the 'thing' that saved. 'Faith alone' makes sense only in the context of 'Christ alone.' 'Faith alone' is the subjective correlate of the objective salvation in Christ alone - it cannot be considered apart from it. To do so is to risk seeing faith as a thing.
Similarly Mike Reeves points out that Martin Luther's favourite phrase for declaring our gracious salvation was not salvation 'by faith alone' but salvation 'by God's Word' alone. Again, faith is not the 'thing' that saves and 'faith alone' is not possession of the single savingly significant substance. (I suspect Luther would have trouble saying this phrase - especially after his fifth Wittenberg ale!).
Faith is, in Anders Nygren's memorable phrase, 'being conquered by the gospel.' Note how passive this image is. Faith is a description of what has happened to the person who's been overwhelmed by Christ in His word. It is not a thing.
Anyway, check out Matt Jenson's article.