Not To Be Confused With
Sola fide concerns the INSTRUMENT of justification (faith, not works); it never means a faith that stays alone — 'faith without works is dead' (James 2). The Reformers taught faith alone justifies, but the faith that justifies is never alone.
The material principle of the Reformation: justification by faith alone through imputed righteousness (Rom 3:28). The full Catholic-Protestant dispute is laid out in the justification question; sola fide is the Protestant pole of it.