... Of this nature is the assertion under the sixteenth head, that the grace of Justification is lost, not only by unbelief, but by any mortal sin. If they meant that we are ejected from the possession (enjoyment) of this great blessing by an evil conscience, I would not at all gainsay them, I mean as far as regards ourselves. For although God does not cast us off, yet an evil conscience is such a separation from him as excludes us from the enjoyment of a lively and justifying knowledge of his paternal love towards us. But they are preposterous, first, in recognising no sin as mortal that is not gross and palpable, whereas most inward sins wound the mind more grievously and even fatally; and, secondly, in not perceiving how a good conscience is the inseparable attendant of faith. Were it not so, how could it be said that our hearts are purified by faith, that Christ dwells in our hearts by faith, that it is the victory by which we overcome the world, the shield for repelling the assaults of the devil, and that we are kept by faith through the power of God unto salvation? (Acts xv.9; Eph.iii.17; I John v.4; Eph. vi.16; I Peter v.9; i.5.) There is no doubt, therefore, that faith is overwhelmed and buried in a man whenever he has been overcome by any temptation so as to abandon the fear of God. For the Spirit of holiness cannot be separated from faith any more than can Christ himself. I do not assert, however, that when we forsake the fear of the Lord faith is altogether extinguished in us. But as the fear of God is oppressed by depraved lusts, so I say that faith is stifled, and for the time exerts its power no more than if it were in a manner dead. ...
Tracts and Letters, Volume 3, 144 (from: Acts of the Council of Trent, Antidote to the Sixth Session)