1Ti 1:18 This charge I commit unto thee, son Timothy, according to the prophecies which went before on thee, that thou by them mightest war a good warfare;
1Ti 1:19 Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck:
1Ti 1:20 Of whom is Hymenaeus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme.
Here, Peter not only says that it is possible for Christians to put away their good conscience, and shipwreck their faith, he names to men who have done so. Hymenaeus and Alexander evidently were former Christians who had blasphemed, and Paul had delivered them to Satan. Here, he warns Timothy to hold to his faith and his good conscience.
This passage and many others would be a ridiculous waste of ink if it were not possible for a Christian to lose his or her faith.