From The Brother’s Karamazov: This was said during a debate about the separation of Church and State.
“I have read the book which you have answered,” he added, addressing Ivan, “and was astounded at the words ‘The Church is a kingdom not of this world. ‘If it is not of this world, then it cannot exist on earth at all. In the Gospel, the words ‘not of this world’ are not used in that sense. To play with such words is indefensible. Our Lord Jesus Christ came to set up the Church upon earth. The Kingdom of Heaven, of course, is not of this world, but in Heaven; but it is only entered through the Church which has been founded and established upon earth. And so a frivolous play upon words in such a connection is unpardonable and improper. The Church is, in truth, a kingdom and ordained to rule, and in the end must undoubtedly become the kingdom ruling over all the earth. For that we have the divine promise.”