The title

The unity of God's kingdom exists in the new heart of one believer.

A heart regenerated by the Word and the Spirit is a new heart, and within it is the unity of God's kingdom.

This is always resisted by the world and those who would spurn the Word and the Spirit to seek a worldly unity.

There are a thousand worldly - man-fearing - justifications for doing this; and among those with such justifications there is even anger when regeneration or new heart is mentioned.

So be it. (Notice you're suddenly very unified when you are angry at one of God's own who tells you things you don't want to hear?)