Jamieson, Fausset and Brown on Matthew 21:37

37. But last of all he sent unto them his son, saying, They will reverence my son--In Mark (Mr 12:6) this is most touchingly expressed: "Having yet therefore one son, His well-beloved, He sent Him also last unto them, saying, They will reverence My Son." Luke's version of it too (Lu 20:13) is striking: "Then said the lord of the vineyard, What shall I do? I will send My beloved Son: it may be they will reverence Him when they see Him." Who does not see that our Lord here severs Himself, by the sharpest line of demarcation, from all merely human messengers, and claims for Himself Sonship in its loftiest sense? (Compare He 3:3-6). The expression, "It may be they will reverence My Son," is designed to teach the almost unimaginable guilt of not reverentially welcoming God's Son.