▼ BDAG 233 s.v. διαμαρτύρομαι 1 has “testify of, bear witness to (orig. under oath)…of repentance to Judeans and Hellenes Ac 20:21.”

▼ Several mss, including some of the more important ones (Ƥ⁷⁴ א Α C [D] E 33 36 323 945 1175 1241 1505 1739 pm and a number of versions), read Χριστόν (Christon, “Christ”) at the end of this verse. This word is lacking in B H L P Ψ 614 pm. Although the inclusion is supported by many earlier and better mss, internal evidence is on the side of the omission: In Acts, both “Lord Jesus” and “Lord Jesus Christ” occur, though between 16:31 and the end of the book “Lord Jesus Christ” appears only in 28:31, perhaps as a kind of climactic assertion. Thus, the shorter reading is to be preferred.

▼ Repentance toward God and faith in our Lord Jesus. Note the twofold description of the message. It is a turning to God involving faith in Jesus Christ.