The Kingship of God is an aspect of His Being that many modern Christians rather lightly skip over. Especially in America, where concepts of individual sovereignty are innate, we do not respond well to the idea that we are complete and total subjects of a King: the Founding Fathers have bred in us a distrust of Kingship.
But God is no "earthly king or potentate," as the Irish-American athlete and flag-bearer at the 1908 Olympic Games in London said. God is the eternal Lord of all creation: His authority is absolute over everything. As the song says, "He's got the whole world in His Hands": he is both a benevolent creator and an absolute monarch. He is the pantokrator, the one who holds all authority and power: in the West we think in terms of omnipotence, but the original Greek term indicates not so much supreme potential as it does supreme and absolute authority. God is King of all and holds all rights and privileges pertaining thereto.
The most important of those rights and privileges is our fealty: we owe our allegiance, above all earthly loyalties, to God and God alone. Should your earthly fealties come in conflict with your heavenly ones, you have but one option: to walk steadfast in the Ways of the Lord. For example, your King commands that you respect all life: any allegiance you have to an earthly power that calls on you to disrespect life or to uphold the rights of others to disrespect life are null and void.
Furthermore, it is on account of God's Kingship that we kneel before Him. Especially when we enter a House of God and come before His True Presence in the Sacrament of the Altar, we bend our knees before Him, acknowledging His preeminence and power over us. Likewise, during the Canon of the Mass, we kneel before him humbly in respect of the great mysteries occurring before our eyes. We, the subjects of the Lord, are unworthy that he should come under our roofs; yet He is a merciful God and has ordained by His mercy and His Word that we partake in His Kingdom.
So swear your fealty to God: swear allegiance before Him, and acknowledge Him always as your King.