Humans are creatures that consist of a body as well as a spirit so that our prayers are not confined to our minds, hearts and voices, but are also expressed by our bodies as well. When our body participates in our prayer, we pray with our whole person. As embodied spirits, this engagement of our entire being in prayer helps us to pray with greater attention.
During Mass we assume different postures: standing, kneeling, sitting, and we are also invited to make a variety of gestures. These postures and gestures are not merely ceremonial. They have profound meaning and, when done with understanding, can enhance our personal participation in Mass. In fact, these actions are the way in which we engage our bodies in the prayer that is the Mass. Each posture we assume at Mass underlines and reinforces the meaning of the action in which we are taking part at that moment in our worship.
Kneeling is one of the most distinctive physical gestures of prayer during the celebration of Mass. In fact, for many centuries the lay faithful of the Roman Rite would kneel for almost the entire duration of Mass. Why is that?
While it's true that standing during prayer was a common posture of the early Christians (and is currently maintained by many Eastern Christians during the Divine Liturgy), kneeling was also part of early Christian tradition. Ratzinger claimed that, "Kneeling does not come from any culture — it comes from the Bible and its knowledge of God." In particular, "Saint Luke, who in his whole work (both the Gospel and the Acts of the Apostles) is in a special way the theologian of kneeling prayer, tells us that Jesus prayed on His knees. This prayer, the prayer by which Jesus enters into His Passion, is an example for us."
Additionally, kneeling is typically seen in the Gospels as a way to express supplication and adoration. Often in the New Testament kneeling is preceded by an act of faith, "I do believe, Lord," and completed by an act of adoration at the majesty of God (cf. John 9:35-38).
Elsewhere, like in many of the healing narratives, the person is presented kneeling in supplication, asking to be healed. For these reasons the Roman Rite instructs the faithful to kneel during Mass specifically when Jesus is made present on the altar.
This physical posture is meant to express a spiritual attitude of adoration before the triune God, truly and substantially present in the Holy Eucharist. It is an act of humility, recognizing our own littleness before the Creator of the world. The act of kneeling prepares our hearts to receive God within our souls, striking down our pride with a physical reminder of what our soul should be like spiritually. In this way, kneeling in the context of the Roman liturgy is directly tied to Jesus' presence in the Eucharist. Kneeling during Mass is an ancient posture, one that expresses a deep spiritual truth that is connected to the Real Presence of Jesus on the altar.