Paul was sincere as a Pharisee
But the example of Saint Paul, if there were no other, is enough to put this beyond all question. He could not only say, when he was a Christian, In this I exercise myself to have always a conscience void of offense toward God and toward men (Acts 24:16). But even concerning the time when he was a Pharisee, Men and brothers, I have lived in all good conscience before God until this day (Acts 23:1). He was therefore sincere when he was a Pharisee as well as when he was a Christian. He was no more a hypocrite when he persecuted the church than when he preached the faith which he once persecuted (Galatians 1:23). Let this then be added to the righteousness of the scribes and Pharisees, a sincere belief that they are righteous, and in all things doing God service.
1771 text
But the example of St. Paul, were there no other, is sufficient, to put this out of all question. He could not only say, when he was a Christian, Herein do I exercise myself to have always a conscience void of offence, toward God and toward men: but even concerning the time when he was a Pharisee; Men and brethren, I have lived in all good conscience before God until this day. He was therefore sincere when he was a Pharisee, as well as when he was a Christian. He was no more an hypocrite when he persecuted the church, than when he preached the faith which once he persecuted. Let this then be added to the righteousness of the Scribes and Pharisees, a sincere belief that they are righteous, and in all things doing God service.