Charles Ellicott Commentary Matthew 26:48

"Now he that betrayed him gave them a sign, saying, Whomsoever I shall kiss, that is he: take him." — Matthew 26:48 (ASV)

Whomsoever I shall kiss — It is probable, from the known customs (1) of the Jews and (2) of the early Christians (Romans 16:16; 1 Thessalonians 5:26), that this was the usual greeting of the disciples to their Master. St. John, it may be noted, makes no mention of the sign, probably because here, as elsewhere, he seeks to provide details that others had overlooked, rather than to repeat what the oral or written teaching of the Church had already made familiar.