A.T. Robertson Commentary Acts 13:24

"when John had first preached before his coming the baptism of repentance to all the people of Israel." — Acts 13:24 (ASV)

When John had first preached (προκηρυξαντος Ιωανου). Literally, John heralding beforehand, as a herald before the king (Luke 3:3). Genitive absolute of first aorist active participle of προκηρυσσω, old verb to herald beforehand, here alone in the N.T., though Textus Receptus has it also in Ac 3:20.

Before his coming (προ προσωπου της εισοδου αυτου). Literally, before the face of his entering in (here act of entrance as 1 Thessalonians 1:9, not the gate as in Heb 10:19). See Mal 3:1 quoted in Mt 11:10 (Luke 7:27) for this Hebrew phrase and also Lu 1:76.

The baptism of repentance (βαπτισμα μετανοιας). 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 Mr 1:4; Luke 3:3. It is clear therefore that Paul understood John's ministry and message as did Peter (Acts 2:38; Acts 10:37).