The First Council of Nicaea (AD 325)had this to say about kneeling on Sundays.
Canon 20. Forasmuch as there are certain persons who kneel on the Lord's Day and in the days of Pentecost, therefore, to the intent that all things may be uniformly observed everywhere(in every parish), it seems good to the holy Synod that prayer be made to God standing.
It's still observed in the Byzantine Church.
Am I reading that right?
I think that it's fine, but to say kneeling during the Eucharistic prayer is a mortal sin, smacks of absolute ignorance and arrogance.
I also think kneeling before the Eucharist brings about humility. IMHO....