The tenth of Paul's Epistles, and the fifteenth book in the New Testament, is I Timothy. It was written around 64 AD to Timothy, one of Paul's closest friends, who he had converted 15 or so years earlier.
The letter was written shortly after Paul had visited Ephesus, where Timothy had been positioned as Paul's representative. Paul hoped to one day return, but this letter was written to give Timothy advice on his ministry in the meantime.
I Timothy 4:1
"Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils"