Albert Barnes Commentary Acts 26:5

"having knowledge of me from the first, if they be willing to testify, that after the straitest sect of our religion I lived a Pharisee." — Acts 26:5 (ASV)

Which knew me. This refers to those who were well acquainted with me.

From the beginning. Anwyen. This means formerly, or from the very start of my career. They were perfectly aware of my entire course of life.

If they would testify. This means if they were willing to bear witness to what they knew.

That after the most straitest. This refers to the most rigid, the most strict way of life, not only concerning the written law of God but also 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. This means a division or party.

I lived a Pharisee. This means I lived in accordance with the rules and doctrines of the Pharisees (see Matthew 3:7 and Acts 22:3 for context).

The reasons why Paul refers to his early life here are:

1. As he had lived during the early period of his life without crime, and 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, showed 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 later time. From the strictness and conscientiousness of his past life, he supposed that Agrippa might argue favorably concerning his subsequent conduct. A virtuous and religious course in early life is usually a sure pledge of virtue and integrity in later years.