Kneeling before someone has always been a sign of humility.
In the Eastern Churches, kneeling is a sign of penance.
While standing does not show humility and at times can be a sign of pride or lack of humility:
It is true that standing does not necessarily show humility, but when properly understood, it shows something else.
Neither position is inherently better, although one or the other might speak to an individual’s spirituality more profoundly.
Here is the explanation from St. Basil the Great (a doctor of the Church) as to why standing is the preferred position for Christian prayer, particularly on Sundays.
“We stand up when praying on the first of the week, though not all of us know the reason. For it is not only that it serves to remind us that when we have risen from the dead together with Christ we ought to seek the things above, in the day of resurrection of the grace given us, by standing at prayer, but that it also seems to serve in a way as a picture of the expected age. Wherefore, being also the starting point of days, though not the first with Moses, yet it has been called the first. For it says: ‘The evening and the morning were the first day’ (Gen. 1:5), on the ground that it returns again and again. The eighth, therefore, is also the first, especially as respects that really first and true eighth day, which the Psalmist too has mentioned in some of the superscriptions of his psalms, serving to exhibit the state which is to succeed this period of time, the unceasing day, the day without a night that follows, the day without successor, the never-ending and unaging age. Of necessity, therefore, the Church teaches her children to fulfill their obligations to pray therein while standing up, in order by constantly reminding them of the deathless life to prevent them from neglecting the provisions for the journey thither. And every Pentecost is a reminder of the expected resurrection in the age to come. For that one first day, being multiplied seven times over, constitutes the seven weeks of the holy Pentecost. For by starting from the first day of the week, one arrives on the same day… The laws of the Church have taught us to prefer the upright posture at prayer, thus transporting our mind, so to speak, as a result of a vivid and clear suggestions, from the present age to the things come in the future. And during each kneeling and standing up again we are in fact showing by our actions that is was through sin that we fell to earth, and that through the kindness of the One Who created us we have been called back to Heaven…” (Canon XCI of St. Basil the Great). The three well-known kneeling prayers of Pentecost composed by this great Father of the Church are thus not read at third hour, when the Holy Spirit descended upon the Apostles, nor at Liturgy on Pentecost, but at vespers, which is already part of the following day, after the Entrance.
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