In biblical theology, the instrumental cause of justification before God is faith alone (faith in Christ’s sacrifice alone).  In other words, faith and only faith is the means by which we are justified.  Faith alone is the instrument by which God justifies us because such faith signifies entire dependence upon the work of Christ and in nothing of our own effort.

Furthermore, if justification by faith alone is denied, then human works must be added to the work of Christ, otherwise, it is not justification by faith alone.  Being justified by faith alone does not mean that this faith that justifies is alone, but that it is accompanied by the person’s regeneration which is wrought by God.  It is this regeneration that brings forth good works.  So, we are justified by faith alone, where faith is the instrumental cause of being justified, but it is accompanied by regeneration which produces good works.