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.

After the most straitest sect - That is, the Pharisees; who were reputed the strictest in their doctrines, and in their moral practices, of all the sects then among the Jews. The sects were the Pharisees, Sadducees, and Essenes.

Which (a) knew me from (b) the beginning, if they would testify, that after the (c) most straitest sect of our religion I lived a Pharisee.
(a) That I was, and where, and how I lived.
(b) That my parents were Pharisees.
(c) The sect of the Pharisees was the most exquisite amongst all the sects of the Jews, for it was better than all the rest.

Which knew me from the beginning,.... From his youth, from his first coming to Jerusalem:
if they would testify; what they know, and speak out the truth of things, they must say,
that after the most straitest sect of our religion I lived a Pharisee; there were three sects of religion among the Jews, the Pharisees, Sadducees, and Essenes; the first of these was the most exact, and came nearest to the truth of doctrine, and was the strictest as to outward holiness of life and conversation, and of this sect the apostle was; and according to it he lived, and that in such a manner, as not to be charged with any notorious crime; and indeed in his own, and very likely in the opinion of others, he was then blameless. See Gill on Matthew 3:7.
(Essenes: A Jewish sect, who, according to the description of Josephus, combine the ascetic virtues of the Pythagoreans and the Stoics with a spiritual knowledge of the divine law. It seems probable that the same name signifies "seer", or "the silent, the mysterious". As a sect the Essenes were distinguished by an aspiration after the ideal purity rather than by any special code of doctrines. There were isolated communities of the Essenes, which were regulated by strict rules, and analogous to those of the monastic institutions of a later date. All things were held in common, without distinction of property; and special provision was made for the relief of the poor. Self-denial, temperance and labour--especially agricultural--were the marks of the outward life of the Essenes; purity and divine communication the objects of aspiration. Slavery, war and commerce were alike forbidden. Their best known settlements were on the north west shore of the Dead Sea. J.B. Smith one volume Bible Dictionary.)

if they would--"were willing to"
testify--but this, of course, they were not, it being a strong point in his favor.
after the most straitest--"the strictest."
sect--as the Pharisees confessedly were. This was said to meet the charge, that as a Hellenistic Jew he had contracted among the heathen lax ideas of Jewish peculiarities.

If they would testify - But they would not, for they well knew what weight his former life must add to his present testimony.