The Apostle Paul's epistle to the Ephesians.
Ephesians is known as one of the "captivity epistles" because Paul indicates he was in prison at the time it was written. Because not all early copies of Ephesians include the address to the audience -- the church in Ephesus -- this letter has often been thought of as a "circular", a letter that was sent to a variety of locations. This may explain the relative lack of personal greetings vs. other Pauline epistles.
This epistle includes some of the very well-known Pauline imagery, including the foundation of the church and the armor of God.