Albert Barnes Commentary Acts 26:4

"My manner of life then from my youth up, which was from the beginning among mine own nation and at Jerusalem, know all the Jews;" — Acts 26:4 (ASV)

My manner of life. My opinions, principles, and conduct.

From my youth. Paul was born in Tarsus; but at an early period, he was sent to Jerusalem for the purpose of education in the school of Gamaliel (Acts 22:3).

Which was at the first. This means from the beginning; it refers to the early part of his youth, the time when opinions and habits are formed.

Know all the Jews. It is not at all improbable that Paul was distinguished in the school of Gamaliel for zeal in the Jewish religion. The fact that he was early entrusted with a commission against the Christians (Acts 9) shows that he was known . He could, therefore, appeal to them regarding the early part of his life; and, doubtless, to the very men who had been his violent accusers.