As Protestants, we say that justification is by faith alone. That little word by...

As Protestants, we say that justification is by faith alone. That little word by is critical to our understanding of how justification takes place. It does not mean that faith is meritorious and obligates God to save us. Rather, the word by indicates grammatically what we call the instrumental dative, which describes the means by which a thing comes to pass.
So, to use Aristotle's categories, faith is the instrumental cause of justification, according to the Protestant view.