Vincent's NT Word Studies
23. They had heard (akouontev hsan). Correlative with I was unknown, verse 22. Note the periphrasis of the participle with the substantive verb, expressing duration. They were hearing all the time that I was thus unknown to them in person.The faith. See on Acts vi. 7, and comp. 2 Thess. iii. 2. The subjective conception of faith as trustful and assured acceptance of Jesus Christ as Savior, tends to become objective, so that the subjective principle is sometimes regarded objectively. This is very striking in the Pastoral Epistles.
Robertson's NT Word Studies
1:23 {They only heard} (monon akouontes esan). Periphrastic imperfect, "They were only hearing from time to time." {That once persecuted us} (ho diwkwn hemas 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} (ten pistin). Here used in the sense of "the gospel" as in #Ac 6:7.