II. They are to be rejected who deny the truth of Christ's flesh, a truth repeated by every recipient at the Holy Eucharist.
Let such men be rejected by the holy members of Christ's Body, and let not Catholic liberty suffer the yoke of the unfaithful to be laid upon it. For they are to be reckoned outside the Divine grace, and outside the mystery of man's salvation, who, denying the nature of our flesh in Christ, gainsay the Gospel and oppose the Creed. Nor do they perceive that their blindness leads them into such an abyss that they have no sure footing in the reality either of the Lord's Passion or His Resurrection: because both are discredited in the Saviour, if our fleshly nature is not believed in Him.
In what density of ignorance, in what utter sloth must they hitherto have lain, not to have learned from hearing, nor understood from reading, that which in God's Church is so constantly in men's mouths, that even the tongues of infants do not keep silence upon the truth of Christ's Body and Blood at the rite of Holy Communion? For in that mystic distribution of spiritual nourishment, that which is given and taken is of such a kind that receiving the virtue of the celestial food we pass into the flesh of Him, Who became our flesh. Hence to confirm you, beloved, in your laudably faithful resistance to the foes of Truth, I shall fully and opportunely use the language and sentiments of the Apostle, and say: “Therefore I also hearing of your faith, which is in the Lord Jesus, and love towards all saints, do not cease to give thanks for you, making mention of you in my prayers that the God of our Lord Jesus Christ, the Father of glory, may give you the spirit of wisdom and revelation in the knowledge of Him, the eyes of your hearts being enlightened that you may know what is the hope of His calling, and what the riches of the glory of His inheritance among the saints, and what is the exceeding greatness of His power in us, who believed according to the working of His mighty power which he has wrought in Christ, raising Him from the dead, and setting Him at His right hand in heavenly places above every principality, and power, and strength, and dominion, and every name which is named not only in this age, but also in that which is to come: and has put all things under His feet, and given Him to be the head over all the Church which is His body, and the fullness of Him Who fills all in all.”
Source: Letters (New Advent)