The loss of faith generally arises from a long continued and willful course of sin, by which faith is weakened until it is unable to hold its ground against the difficulties of skepticism, or of its own accord fades away and disappears. No calamity in the whole world is so terrible as this. Woe to those who through sin lose their faith! Yet there are souls without number to whom this terrible calamity has befallen. Nothing but a miracle of mercy can save them. We must pray God for such, that He may bring them to the truth before it is too late.
The loss of faith is always the result of pride in one shape or another. Nothing is so subversive of faith as pride. The proud man hates submission and loves to choose his own path. Faith demands submission, and marks out our path for us. Faith requires that we should lean upon God. Pride will lean upon none other than self. Faith requires loyalty; the very essence of pride is rebellion against God. How anxious then must I be to humble myself in order that I may preserve my faith.
All sins, of whatever kind, tend to gradually destroy our faith. Faith is a willing dependence upon God's authority, and all sins, even venial sins, involve a disregard of His authority and an independence of Him. Faith, then, if it is to be strong in its apprehension of Divine things, must be based on a continual obedience to God. If we love obedience, and are careful to avoid all that displeases God, we need never fear the loss of our faith.
- text from Beautiful Pearls of Catholic Truth; it has the Imprimatur of Archbishop Michael Augustine Corrigan, Diocese of New York, 6 October 1897