John 3:3 Jesus answered and said to him, Verily, verily, I say to thee, Except a man be born again, he cannot see the kingdom of God.

The only way to be able to see the Kingdom of the Heavens is to be born again.  Those who have not been born again are largely blind to reality.  They cannot see on the level of spirit.  They cannot possibly understand the natural things they are seeing because they have no frame of reference.  Most of them think that there is no spiritual realm.  When a person is born again, that person receives spiritual senses.  When they are obedient and submissive to the Spirit of God, those spiritual senses become more acute.  They become more accurate.  If they are not submissive to the Spirit of God, those spiritual senses become dull.  The Christian becomes calloused against God.

In John 18:36 Jesus said that His Kingdom was not of this world, or not of this worldly order.  This is the Kingdom of the Heavens and cannot be seen unless a person has spiritual eyes to see.