Atos 18:12

But when Gallio was proconsul of Achaia, the Jews with one accord rose up against Paul and brought him before the judgment-seat, saying, This man persuadeth men to worship God contrary to the law.

Gallio ... This man was the older brother of the philosopher Seneca, advisor to Nero. He was born Marcus Annaeus Novatus; but upon being adopted by a rich man, Lucius Junius Gallio, he took the full name of "Lucius Junius Gallio Annaeus."[20] A fragment discovered at Delphi in 1905 marks the beginning of this man's proconsulship as 51 A.D.; but as Jack P. Lewis observed:

We do not know how long he was proconsul, nor for certain whether Paul was brought before him at the beginning or end of his term. It is assumed that Gallio was proconsul only one year and that Paul was arraigned at the beginning of his term, giving the conclusion that Paul arrived in Corinth in 49-50 A.D.[21]

Recent archeological discoveries support the earlier date for First Thessalonians, as advocated by Harnak, as 48-49 A.D.

This man persuadeth men to worship God contrary to the law ... This means "contrary to Jewish law"; but as Judaism was a legal religion in Rome, the implication is that Christianity was not. These Jews were not nearly as clever in their opposition as those in Thessalonica; and their efforts were quickly suppressed.

The usual assumption is that Gallio, who amiable disposition was widely acclaimed, no sooner entered into his proconsulship than the Jews descended upon him demanding action against Paul.

Paul was all set to defend himself against the Jewish charges, but before he ever opened his mouth, Gallio gave sentence in his favor and dispersed the accusers.

[20] Jack P. Lewis, Historical Backgrounds of Bible History (Grand Rapids, Michigan: Baker Book House, 1971), pp. 153-155.

[21] Ibid.