Kingdom (Βασίλειο in Greek) refers to the “kingship” or presence of the King, not a physical place. Since God is King of the Universe, of all things visible and invisible, His Kingdom does not need to be “built” in the sense of construction’ it already exists everywhere. It is our responsibility to build the Kingdom by populating it with subjects of The Lord. When we have children and raise them in the Faith, that is what we are doing: Building the Kingdom of God.