Preach, saying This is the preaching, [1] I spoke of, by which Christ intended to arouse the minds of the nation to expect an approaching redemption. The kingdom of heaven is at hand For the kingdom of heaven Luke substitutes the kingdom of God; but the meaning is the same. It was to inform the Jews, first, that they owed their restoration to divine agency, and not to the kindness of men; secondly, that under the reign of God their condition would be prosperous; and, thirdly, that the happiness which had been promised to them was not earthly and fading, but heavenly and eternal.

Footnotes

1 - "La predication, ou publication;" -- "the preaching, or publication."

The kingdom of heaven is at hand - Or, more literally, the "reign" of heaven, or of God, draws near. See the notes at Matthew 3:2.

And as ye go, preach - πορευομενοι δε κηρυσσετε, and as you proceed, proclaim like heralds - make this proclamation wherever ye go, and while ye are journeying. Preach and travel; and, as ye travel, preach - proclaim salvation to all you meet. Wherever the ministers of Christ go, they find lost, ruined souls; and, wherever they find them, they should proclaim Jesus, and his power to save. For an explanation of the word proclaim or preach, see on Matthew 3:1 (note).
From this commission we learn what the grand subject of apostolic preaching was - The Kingdom Of Heaven Is At Hand! This was the great message. "They preached," says Quesnel, "to establish the faith; the kingdom, to animate the hope; of heaven, to inspire the love of heavenly things, and the contempt of earthly; which is at hand, that men may prepare for it without delay."

(2) And as ye go, preach, saying, The kingdom of heaven is at hand.
(2) The sum of the gospel, or preaching of the apostles.

And as ye go,.... Through the cities of Judea, and the streets thereof, from one city to another, from place to place; for these were itinerant preachers, who were not to abide long in any place, but to move about, that the Gospel might be spread all over the land, and the lost sheep in every corner be sought out and found.
Preach, saying, the kingdom of heaven is at hand. This was to be the subject matter of their ministry, which they were to proclaim aloud in every place; and which is expressed in the same words with which John the Baptist, and Christ himself, began their ministry,
Matthew 3:2 which shows the entire harmony, and strict agreement, there were between them: for the meaning of the phrase; see Gill on Matthew 3:2. The Cambridge copy reads, "repent, for the kingdom", &c.

And as ye go, preach, saying, The kingdom of heaven is at hand--(See on Matthew 3:2).

Preach . . . The kingdom of heaven is at hand. John the Baptist, and Christ also, had preached, "The kingdom is at hand." It had not yet been inaugurated. So the apostles were still to preach. It was near, but not in existence. There was no such charge in the second commission. Then "all power in heaven and in earth was in the hands of Christ." He became King after he suffered, and his kingdom was inaugurated on earth on the day of Pentecost. When he was "lifted up" (John 3:15), he became King.