The Hebrew Scriptures at the foundation of our faith often speak of God as a king. But the king is a very poor model for thinking about God. Unlike the Old Testament writers, Jesus never uses it. The closest he comes is in talking about the kingdom of God, but the phrase has the meaning of moral authority rather than a physical place or political arrangement, and Jesus is always teaching about the nature of that authority, and always contrasting it with our normal understanding.
The models Jesus uses instead to guide our reflection are the father and the shepherd. The quality those models share is the obligation of self-sacrifice, and that is the opposite of what defines a king (or a dictator, or an emperor, or a president). The quality that defines a king is that others are obligated to him for self-sacrifice.
When we kneel in prayer, we are not kneeling in deference to power. We kneel in deference to weakness. This is the fundamental mystery of Christianity (and what Nietzsche, who fully understood Christianity, railed against): it worships weakness. We submit our will, not to a more powerful will, but to a will that abjures power altogether, and asks us to do the same. Asks, not orders. When we kneel in prayer, we are not signaling our acceptance of a command or a ranking, but our acceptance of an invitation to serve, rather than be served.
Jesus knelt to wash the disciples’ feet.