A.T. Robertson 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)

They only heard (μονον ακουοντες ησαν). Periphrastic imperfect, "They were only hearing from time to time."

That once persecuted us (ο διωκων ημας ποτε). Present active articular participle, a sort of participle of antecedent time suggested by ποτε, "the one who used to persecute us once upon a time."

The faith (την πιστιν). Here used in the sense of "the gospel" as in Ac 6:7.