The Apostle Paul's epistle to the Galatians.
Paul wrote to the Galatians among whom he had labored as a missionary, to call them back to the gospel he had preached to them--they (or at least some of them) had apparently been led astray by those who believed the law of Moses should still be kept in its entirety.
Galatians provides a wealth of detail--from Paul himself--about key events in his life as a Christian.
Various views exist as to the date of the composition of this epistle. The themes speak strongly to the topics reviewed at the Jerusalem Conference of Acts 15 (circa AD 49), and many hold that Galatians was written either just before or just after the conference.
Others consider the similarity between Galatians and Romans to indicate that the two epistles were written around the same time. Paul's epistle to the Romans was written while he was wintering in Corinth circa AD 56-57.