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 6:13-16
"I give thee charge in the sight of God, who quickeneth all things, and before Christ Jesus, who before Pontius Pilate witnessed a good confession; that thou keep this commandment without spot, unrebukeable, until the appearing of our Lord Jesus Christ: which in his times he shall shew, who is the blessed and only Potentate, the King of kings, and Lord of lords; who only hath immortality, dwelling in the light which no man can approach unto; whom no man hath seen, nor can see: to whom be honour and power everlasting. Amen"