Trent on Justification - Canon Nine

Trent’s ninth canon on justification condemns the Protestant error of “sola fide,” and the error among the Reformed that free assent is required.

If any one saith, that by faith alone the impious is justified; in such wise as to mean, that nothing else is required to co-operate in order to the obtaining the grace of Justification, and that it is not in any way necessary, that he be prepared and disposed by the movement of his own will; let him be anathema.

This is not contrary to salvation by grace alone, Protestant objections notwithstanding. There is an unwarranted assumption held by most Protestants that human cooperation in justification runs counter to salvation by grace alone, but we've seen that this is false with respect to the Catholic doctrines of grace: we must consent, and we must “work out our salvation in fear and trembling,” but our very ability to do these things is necessarily preceded by grace.