9 For faith and godliness are allied to each other, and sisters; and he who believes in Him is godly, and he also who is godly, believes the more. He therefore who is in a state of wickedness, undoubtedly also wanders from the faith; and he who falls from godliness, falls from the true faith. Paul, for instance, bearing testimony to the same point, advises his disciple, saying, 'Avoid profane conversations; for they increase unto more ungodliness, and their word takes hold as does a canker, of whom are Hymenæus and Philetus.'
In what their wickedness consisted he declares, saying, 'Who have erred from the faith, saying that the resurrection is already past. ' But again, desirous of showing that faith is yoked with godliness, the Apostle says, 'And all those who will live godly in Jesus Christ shall suffer persecution. ' Afterwards, that no man should renounce godliness through persecution, he counsels them to preserve the faith, adding, 'You, therefore, continue in the things you have learned, and hast been assured of. ' And as when brother is helped by brother, they become as a wall to each other; so faith and godliness, being of like growth, hang together, and he who is practised in the one, of necessity is strengthened by the other.
Therefore, wishing the disciple to be exercised in godliness unto the end, and to contend for the faith, he counsels them, saying, 'Fight the good fight of faith, and lay hold on eternal life.' For if a man first put away the wickedness of idols, and rightly confesses Him Who is truly God, he next fights by faith with those who war against Him.
Source: Letters (New Advent)