Even if kneeling were a construct of feudal relations, that would not change the fact that in our culture kneeling is an expression of humility. But the truth is that kneeling as an act of submission goes back far earlier than the Byzantine court. We find it both in the Old Testament and in the New. The old Catholic Encyclopedia
When the occasion was one of special solemnity, or the petition very urgent, or the prayer made with exceptional fervor, the Jewish suppliant knelt. Besides the many pictorial representations of kneeling prisoners and the like, left us by ancient art, Genesis 41:43 and Esther 3:2 may be quoted to show how universally in the East kneeling was accepted as the proper attitude of suppliants and dependents. Thus Solomon dedicating his temple “kneeling down in the presence of all the multitude of Israel, and lifting up his hands towards heaven.” And: “Of Christ’s great prayer for his disciples and for his Church we are only told that ‘lifting up his eyes to heaven, he said,’” etc. (John 17:1); but of his agony in the garden of Gethsemane: “kneeling down, he prayed” (Luke 22:41). The lepers, beseeching the Savior to have mercy on them, kneel (Mark 1:40; cf. 10:17).