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. Compare Philippians 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.