Adam Clarke on Acts 15:23

Send greeting unto the brethren - of the Gentiles - There was no occasion to send such a letter to the brethren which were of the Jews, because that law which had been so long read in their synagogues taught them all those things; and therefore the epistle is sent exclusively to the Gentiles. The word greeting is in the original χαιρειν, to be well, to be safe; a very usual form in Greek epistles, the word ευχομαι being understood, I wish thee to be well.