Therefore whenever you hear of the kingdom of heaven, you should not merely gaze up to heaven, but look around you upon the earth and seek it among the people, in the whole world, where the Gospel is taught and Christ is believed in, and the Sacraments are properly used. The “kingdom of heaven,” in plain language, is the kingdom of Christ and the kingdom of the Gospel and of faith.
For wherever the Gospel is there Christ is also. And where Christ is there is the Holy Ghost also and His kingdom, the true kingdom of heaven. And all who have the Word and Sacraments and believe and remain in Christ by faith are heavenly princes and children of God. All that remains to be done is that God remove the partition which still separates us, that is, that we die, then all will be heaven and salvation.
Learn to understand then, in the first place, that the kingdom of heaven is the kingdom of our Lord Jesus and is to be found wherever the Word and faith are. In this kingdom we have life in hope and are, according to the Word and faith, cleansed from all sin and delivered from death and hell, notwithstanding the old Adam and the corruption of the flesh. The old Adam has not yet been destroyed and the sinful flesh has not been put away; this must yet be done, then there shall be nothing but life, righteousness and salvation. (Luther’s House Postil, Sermon for the Twentieth Sunday after Trinity, Volume 3, p. 843)