We are ambassadors for Christ and His kingdom is not of this world.
Acts 28:23 And when they had appointed him a day, there came many to him into his lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and out of the prophets, from morning till evening.
What he preached. He does not fill their heads with curious speculations, nor with matters of state and politics, but he keeps to his text, minds his business as an apostle.
(1.) He is God’s ambassador, and therefore preaches the kingdom of God, does all he can to preach it up, negotiates the affairs of it, in order to the advancing of all its true interests. He meddles not with the affairs of the kingdoms of men; let those treat of them whose work it is. He preaches the kingdom of God among men, and the word of that kingdom; the same that he defended in his public disputes, testifying the kingdom of God (v. 23), he enforced in his public preaching, as that which, if received aright, will make us all wise and good, wiser and better, which is the end of preaching.
(2.) He is an agent for Christ, a friend of the bridegroom, and therefore teaches those things which concern the Lord Jesus Christ-the whole history of Christ, his incarnation, doctrine, life, miracles, death, resurrection, ascension; all that relates to the mystery of godliness. Paul stuck still to his principle-to know and preach nothing but Christ, and him crucified. Ministers, when in their preaching they are tempted to diverge from that which is their main business, should reduce themselves with this question, What does this concern the Lord Jesus Christ? What tendency has it to bring us to him, and to keep us walking in him? For we preach not ourselves, but Christ.