All that he did had the following purpose. He wished to show that he could never have become renowned among those whom he had formerly persecuted, unless his preaching had been approved by the judgment of those very persons who had earlier known his wickedness, eum malum noverant.
He quietly returns to this purpose, affirming that in Judea he labored for so short a time that the believers did not even know him by sight. He quietly returns to this purpose, affirming that in Judea he labored for so short a time that the believers did not even know him by sight.
By this he shows that neither Peter, nor James, nor John were his teachers, but Christ, who revealed the Gospel to him.
At the same time one must note that earlier he is presented as a persecutor of the Church, but here as a persecutor of the faith. There it is people; here it is the work itself.
Thus now, or then, that is, tunc instead of nunc, it is more fittingly said: 'He preaches the faith which he once tried to root out.' Indeed, this could not have been said in the same way about the Church.