Galatians 1:13
For ye have heard of my conversation in time past in the Jews’ religion, how that beyond measure I persecuted the church of God, and wasted it: For ye have heard of my conversation in time past in the Jews’ religion, how that beyond measure I persecuted the church of God, and wasted it:
This is Paul's own testimony given after his conversion, addressed to the church in Galatia, recalling his former life in the Jews' religion. Before that conversion he was deeply committed to Judaism as a rigid and intolerant Pharisee, advancing beyond many of his equals in his nation through exceedingly great zeal for the traditions of his fathers. In that zeal he opposed Jesus and persecuted the church of God beyond measure, seeking to devastate, waste and destroy its followers.
He reviewed this former life and early Christian experience to demonstrate that a special manifestation of divine power had brought him to understand the gospel. That account supported his claim that his teaching came by revelation and instruction from God and Christ rather than from people or human teachers. His encounter with Christ as revealed to him and recorded in Acts was the turning point where transforming mercy changed him from a persecutor into a sincere penitent, fervent believer and apostle by God's will. He then abandoned the honors and advantages he had expected in order to devote his life to preaching the gospel, and this radical change of course shows that he was not acting from worldly or selfish motives. Afterward, when he himself had to suffer under the same will of men whose hostility had hated Christ, he came to understand the nature of the persecuting work he had once done.