If any one saith, that a man once justified can sin no more, nor lose grace, and that therefore he that falls and sins was never truly justified; or, on the other hand, that he is able, during his whole life, to avoid all sins, even those that are venial,-except by a special privilege from God, as the Church holds in regard of the Blessed Virgin; let him be anathema.
The Christian can certainly lose his justification; else a passage like (for example) Hebrews 6:4f. makes no sense at all: who else but real Christians ever partakes of the Holy Spirit? Or who else has been illuminated, or tasted the heavenly gift?
But the fact that we may reject God’s gift doesn’t mean that it is any less a gift. We cannot save ourselves. It is not credible in the least to say that choosing to believe in a freely offered salvation is somehow “works-based,” but that the act of faith intended by the Protestant’s “sola fide” is not. We are saved by grace; the fact that we may reject that grace doesn’t make our acceptance of it a saving act.