Thank you for the question, “When did the Apostle Paul learn Greek?”
The Scriptures does not state when Saul of Tarsus learned Greek. Since Saul was born in Tarsus of Cilicia (Acts 22:2), it is likely that he learned Greek as a child as this was a Greek-Roman city. It is may well be that he learned the Septuagint of the Old Testament while living in Tarsus and Hebrew under Gamaliel. The Apostle Paul was fluent in both Greek and Hebrew, able to read and write in both languages.
Did Saul of Tarsus had the gift of learning languages? Perhaps that was was one of the reasons why he was sent to Jerusalem to learn under the great teacher, Gamaliel. From the citations by Dr. Luke, the Apostle Paul was proficient in Greek to debate the Greek philosophers and well aware of the Greek-Roman culture. His travels increased his awareness of the Greek-Roman world.
SUMMARY: It is likely that Saul of Tarsus learned Greek as he studied the Septuagint and later with Gamaliel in Hebrew. His childhood in Tarsus enable him to adjust to both cultures in which he can preached and teach in both Greek and Hebrew.
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