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

tc Several mss, including some of the more important ones (𝔓74 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.

sn 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.