Which knew me - Who were well acquainted with me.
From the beginning - ἄνωθεν anōthen. Formerly; or from the very commencement of my career. Who were perfectly apprised of my whole course.
If they would testify - If they would bear witness to what they know.
That after the most straitest - The most rigid; the most strict, not only in regard to the written Law of God, but to the traditions of the elders. Paul himself elsewhere testifies Philippians 3:4-6 that he had enjoyed all the advantages of birth and training in the Jewish religion, and that he had early distinguished himself by his observance of its rites and customs.
Sect - Division or party.
I lived a Pharisee - I lived in accordance with the rules and doctrines of the Pharisees. See the notes on Matthew 3:7. The reasons why Paul here refers to his early life are:
(1) As he had lived during the early period of his life without crime; as his principles had been settled by the instruction of the most able of their teachers, it was to be presumed that his subsequent life had been of a similar character.
(2) As he, at that period of his life, evinced the utmost zeal for the laws and customs of his country, it was to be presumed that he would not be found opposing or reviling them at any subsequent period. From the strictness and conscientiousness of his past life, he supposed that Agrippa might argue favorably respecting his subsequent conduct. A virtuous and religious course in early life is usually a sure pledge of virtue and integrity in subsequent years.