Matthew 3:2

λέγων introduces the burden of his preaching. μετανοεῖτε, Repent. That was John's great word. Jesus used it also when He began to preach, but His distinctive watchword was Believe. The two watchwords point to different conceptions of the kingdom. John's kingdom was an object of awful dread, Jesus' of glad welcome. The message of the one was legal, of the other evangelic. Change of mind John deemed very necessary as a preparation for Messiah's advent. ἡ βασιλεία τῶν οὐρανῶν, the Kingdom of Heaven. This title is peculiar to Matthew. In the other Gospels it is called the Kingdom of God. Not used either by John or by Jesus, says Weiss, but to be ascribed to the evangelist. There does not seem to be any urgent reason for this judgment. In Daniel 2:44 the kingdom is spoken of as to be set up by “the God of heaven,” and in the Judaistic period previous to the Christian era, when a transcendent conception of God began to prevail, the use of heaven as a synonym for God came in. Custom might cause it to be employed, even by those who did not sympathise with the conception of God as transcendent, outside and far off from the world (vide note in H. C., p. 55).