When John had first preached (prokēruxantos Iōanou). Literally, John heralding beforehand, as a herald before the king (Luk 3:3). Genitive absolute of first aorist active participle of prokērussō, old verb to herald beforehand, here alone in the N.T., though Textus Receptus has it also in Act 3:20.
Before his coming (pro prosōpou tēs eisodou autou). Literally, before the face of his entering in (here act of entrance as 1Th 1:9, not the gate as in Heb 10:19). See Mal 3:1 quoted in Mat 11:10 (Luk 7:27) for this Hebrew phrase and also Luk 1:76.
The baptism of repentance (baptisma metanoias). Baptism marked by, characterized by (genitive case, case of kind or species) repentance (change of mind and life). The very phrase used of John’s preaching in Mar 1:4; Luk 3:3. It is clear therefore that Paul understood John’s ministry and message as did Peter (Act 2:38; Act 10:37).