βασιλεία τῶν οὐρανῶν

The Greek phrase βασιλεία τῶν οὐρανῶν consists of βασιλεία (basileia) and οὐρανός (ouranos). The former means, according to the lexicon, “royal power, kingship, dominion, rule - not to be confused with an actual kingdom but rather the right or authority to rule over a kingdom.”

A kingship or dominion offers protection and shelter to the subjects of the realm. Thus “shelter” or “protection” would be appropriate translations in this context. These indicate "sanctuary" - that consciousness where one takes refuge in God.

This is further explained in the endnote for βασιλεία (basileia).

The latter word - οὐρανός (ouranos) refers to, “the region above the sidereal heavens, the seat of order, of things eternal and consummately perfect where God dwells and other heavenly beings” according to Thayer’s lexicon.

Thus the most appropriate phrase to capture this teaching by John and Jesus would be the “sanctuary of the spiritual realm.”