“‘My kingdom is not of [ek: out from] this world,'” is a statement about the source — not the nature — of His reign, as the epexegetical ending of the verse makes obvious: ‘My kingdom is not from here [enteuthen].’ The teaching is not that Christ’s kingdom is wholly otherworldly, but rather that it originates with God Himself (not any power or authority found in creation.”
Dr. Greg Bahnsen
God & Politics — pg. 27
B. F. Wescott speaking of John 18:36 could comment,
“Yet He did claim a sovereignty, a sovereignty of which the spring and the source was not of earth but of heaven. My Kingdom is not of this world (means it) does not derive its origin or its support from earthly sources.”
The Gospel According To John — pg. 260
John 18:36 along with Matthew 22:15-22 are two of the passages that are often put forth as defeaters for the comprehensive sovereignty of the Lord Jesus over this world. Bahnsen clearly shows here, quite in agreement with the Greek scholar B. F. Westcott, that God’s Kingdom, as it manifests itself in this world, is energized by a source outside this world. This is important to emphasize because many people read John 18:36 as proof that the Kingdom of Jesus does not and should not express itself in this world. Often this verse is appealed to in order to prove that God’s Kingdom is only “spiritual” and as such Christians shouldn’t be concerned about what are perceived as “non-spiritual” realms. Support for such thinking, if there is any, must come from passages other than John 18:36.