A.T. Robertson 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)

Gave them a sign (εδωκεν αυτοις σημειον). Probably just before he reached the place, though Mark has "had given" (δεδωκε) which certainly means before arrival at Gethsemane. At any rate Judas had given the leaders to understand that he would kiss (φιλησω) Jesus in order to identify him for certain. The kiss was a common mode of greeting and Judas chose that sign and actually "kissed him fervently" (κατεφιλησεν, verse 49), though the compound verb sometimes in the papyri has lost its intensive force. Bruce thinks that Judas was prompted by the inconsistent motives of smouldering love and cowardice. At any rate this revolting ostentatious kiss is "the most terrible instance of the εκουσια φιληματα εχθρου (Proverbs 27:6)," the profuse kisses of an enemy (McNeile). This same compound verb occurs in Lu 7:38 of the sinful woman, in Lu 15:20 of the Father's embrace of the Prodigal Son, and in Ac 20:37 of the Ephesian elders and Paul.