John Calvin Commentary Galatians 1:23

"but they only heard say, He that once persecuted us now preacheth the faith of which he once made havoc;" — Galatians 1:23 (ASV)

Which once he destroyed. This does not mean that faith may actually be destroyed, but that he lessened its influence on the minds of weak men. Besides, it is the will, rather than the deed, that is here expressed.