"Another fundamental right of the faithful, as noted in canon 213, is "the right to receive assistance by the sacred Pastors from the spiritual goods of the Church, especially the word of God and the Sacraments". In view of the law that "sacred" ministers may not deny the sacraments to those who opportunely ask for them, are properly disposed and are not prohibited by law from receiving them" (canon 843 § 1), there should be no such refusal to any Catholic who presents himself for Holy Communion at Mass, except in cases presenting a danger of grave scandal to other believers arising out of the person's unrepented public sin or obstinate heresy or schism, publicly professed or declared. Even where the Congregation has approved of legislation denoting standing as the posture for Holy Communion, in accordance with the adaptations permitted to the Conferences of Bishops by the Institution Generalis Missalis Romani n. 160, paragraph 2, it has done so with the stipulation that communicants who choose to kneel are not to be denied Holy Communion on these grounds. "
... Congregation de Cultu Divino et Disciplina Sacramentorum Rome, July 1, 2002