Saint Leo I (Church Father)

“When the Lord says: ‘Unless you shall have eaten the flesh of the Son of Man and shall have drunk His blood, you shall not have life in you,’ you ought to so communicate at the Sacred Table that you have no doubt whatever of the truth of the Body and the Blood of Christ.  For that which is taken in the mouth is what is believed in faith; and in doing those respond, ‘Amen,’ who argue against that which is received.” (Sermons, [91, 3], c. 461)
 
(Cf. Michael Cuozzo, Theological Aspects of Worshipping God in the Holy Eucharist (Unpublished doctoral dissertation). Northwestern Theological Seminary, Florida, 2006), p. 25)