Albert Barnes on Acts 26:4

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

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

Which was at the first. Which was from the beginning; the early part of which, the time when the 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. Comp. Php 3:4-6. He might appeal to them, therefore, in regard to the early part of his life; and, doubtless, to the very men who had been his violent accusers.

(c) "manner of life" 2Ti 3:10