They only heard (monon akouontes ēsan). Periphrastic imperfect, “They were only hearing from time to time.”
That once persecuted us (ho diōkōn hēmas pote). Present active articular participle, a sort of participle of antecedent time suggested by pote, “the one who used to persecute us once upon a time.”
The faith (tēn pistin). Here used in the sense of “the gospel” as in Act 6:7.