To preach the Kingdom is to preach the Gospel and not the doctrines of men

"The first such text Luke 10:16, where Christ says, 'He who hears you, hears me; and he who rejects you, rejects me.' He says the same thing in Matthew 10:40 and in Mark 6:11. Here, they claim, Christ demands of us that we accept their man-made laws.
I reply: That is not true. For immediately before speaking these words, Christ says, 'Go your way and say, the kingdom of God is at hand.' With these words Christ stops the mouths of all the teachers of the doctrines of men and instructs the apostles what they are to teach. He himself puts the word into their mouth, saying that they shall preach of the kingdom of God. Now he who does not preach of the kingdom of God is not sent by Christ, and to him these words do not apply. Rather do these words demand of us that we should not listen to the doctrines of men.
Now to preach of the kingdom of God is nothing else than to preach the gospel, in which is taught the faith of Christ by which alone God dwells and rules in us. But the doctrines of men do not preach about faith, but about eating, clothing, times, places, persons, and about purely external matters which are of no profit to the soul.
Now look at the pious shepherds and faithful teachers, how honestly they have dealt with the poor common people. This text, 'Who hears you, hears me,' they have in a masterful fashion torn out of its context and have terrified us with it, until they have subjugated us unto themselves. But what precedes, 'Preach about the kingdom of God,' they have taken good care not to mention. Bravely have they leaped over it, so that by no means should they be compelled to preach only the Gospel. These noble and most excellent teachers! And we are even supposed to thank them in addition!
Again, in Mark, the last chapter [16:15], where Christ was sending out the disciples to preach, let us hear how he gives them commandment, sets a limit to their teaching, and bridles their tongues when he says, 'Go into all the world and preach the gospel to every creature. He who believes will be saved,' etc. He does not say, 'God and preach what you like, or what you think to be right.' But he puts his own word into their mouth and bids them preach the gospel.
He says the same thing again in the last chapter of Matthew [28:19], 'God and teach all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you.' See, here again he does not say, 'Teach them to observe what you invent,' but what I have commanded you. Therefore it cannot be otherwise: the pope and his bishops and teachers must be wolves and apostles of the devils, for they teach not the commands of Christ, but their own words.
So also in Matthew 25[:14-15], in the parable of the three servants, the Lord points out that the householder instructed the servants to trade not with their own property but with his, and gave the first five talents, the next two, and the third one." - Martin Luther, Avoiding the Doctrines of Men, Pg. 223 - 224