Thinking About Faith Alone and Christ Alone, Part 1
Sole fide means "faith alone." It refers to the truth that our justification, grounded in Christ's work, comes to us through faith alone, and through no other means. There are various ways this is explained. It is said that we lay hold of Christ's work on our behalf through faith alone, that faith is the conduit through which Christ's work comes to us, or that faith is the instrument or vessel which receives God's gift of salvation. I think those are all good descriptions of the role faith plays in our justification.
If we are justified through the means of faith alone, it is certainly right to say that faith is necessary for justification, since it's through faith that we receive it; but at the same time, faith is not the grounds for our justification--the basis upon which we can be justified--since the grounds or basis for our salvation is Christ's work alone, not Christ's work plus our faith. Scripture always speaks of faith, in relation to justification, in language that refer to means only, and never uses the language of grounds in relation to it. That's why thoughtful theologians say salvation is by or through faith, but they don't say it is because of faith: they are being careful to maintain the distinction that faith is the means of salvation, but not the grounds of it.
Are you not sure of the distinction? Here's an example that might help. Imagine a parent punishing a child with good cause. When we use the term "with good cause", we are using the language of grounds. We are saying that there is suitable grounds or sufficient reason for the punishment. The child is punished because he or she did something that needed punishment, and that misdeed is the grounds--or the good cause--for the punishment. Let's say, to expand the illustration, that the punishment was the removal of a certain privilege. Going without the privilege is the way the punishment is received by the child. It's not the grounds of the punishment, which is the misdeed, as discussed above, but the means by which the punishment comes. Going without the privilege is, of course, necessary in order for the child to be punished, but we can't say it's the reason or cause or grounds for the punishment.*
So the answer to the child's question, "Why was I punished?" is not "Because you went without the X-box for a day," but "Because you clobbered your brother over the head with your controller." The answer to the question "How was I punished?" is "By going without the X-box." Because, why, cause, reason, and basis are word in English that speak to grounds; how, by, and through speak of means.
(I know we don't always use language in exactly this way or this precisely. For example, we also use cause to refer to something that brings about an event, which speaks more to agency**, and, of course, that's a right way to use the term. However, in the realm of moral, legal, or theological discussion, the word cause is used frequently as a synonym for reason or grounds, and that's the way I'm using it here. The reason it's so important to think through the terms we use is that when we're speaking about important things--like our salvation, for instance--where much can be gained from understanding the relationships between the necessary elements, precise language helps us understand the concepts without confusion.)
The relationship between in Christ alone and through faith alone in our salvation is similar to the relationship between the child's misdeed and the loss of the X-box in our example. It is similar in that the grounds, reason or cause of salvation is Christ alone, and the means through which our salvation comes is faith alone, so that the answer to the question, "Why am I saved?" is "Because of Christ's work on my behalf," rather than "Because I believed." Faith comes in as an answer to the question "How am I saved?"; I am saved "through faith alone."
The distinction between faith as the means of salvation and Christ's work as the grounds of salvation is the background for the rest of what I want to write on this subject. Next up, I'll post on the reason that faith alone as the means of salvation fits so perfectly with Christ alone as the grounds of salvation. After that, I plan to post on some sloppy theological thinking that adds faith to Christ's work as the grounds of salvation.
*This illustration doesn't work perfectly, but it's the best I can think of, so I'm sticking with it for now.
**Faith is not the agent of salvation, either, by the way. But that's a whole nuther discussion.