Matthew 10:7

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."

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.