Macmillan wrote about it in his book.
Thus we see that those of the temple class are clearly designated as the Lord's witnesses at this time, to bring a message of consolation to the people, that the Kingdom of heaven is here, and that millions now living will never die. Thus it is seen that God purposes that his name shall be magnified, that the people shall know that he is Lord. Thus we see that God purposes to have a people in the earth in this time of stress, clearly marked as separate and distinct from all others, standing as his witnesses, fearlessly crying out the message: "The kingdom of heaven is at hand!" . . .
For six thousand years God has been preparing for this kingdom. For nineteen hundred years he has been gathering out the kingdom class from amongst men. Since 1874 the King of glory has been present; and during that time he has conducted a harvest and has gathered unto himself the temple class. Since 1914 the King of glory has taken his power and reigns. He has cleansed the lips of the temple class and sends them forth with the message. The importance of the message of the kingdom cannot be overstated. It is the message of all messages. It is the message of the hour. It is incumbent upon those who are the Lord's to declare it. The kingdom of heaven is at hand; the King reigns; Satan's empire is falling; millions now living will never die.... The world must know that Jehovah is God and that Jesus Christ is King of kings and Lord of lords. This is the day of all days. Behold, the King reigns! You are his publicity agents. Therefore advertise, advertise, advertise, the King and his kingdom!
Suddenly, as these words filled the auditorium, a platform length banner was unfurled that echoed the stirring phrase: "Advertise the King and Kingdom."
A mighty shout went up from the assembly. Here was our answer.