The Most Holy Eucharist

The doctrine of the Real Presence asserts that in the Holy Eucharist, Jesus is literally and wholly present—body and blood, soul and divinity—under the appearances of bread and wine. (cf. 1 Cor. 10:16–17, 11:23–29; and, most forcefully, John 6:32–71). The early Church Fathers interpreted these passages literally. Below are the teachings of the early church Fathers:

Ignatius of Antioch
"I have no taste for corruptible food nor for the pleasures of this life. I desire the bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire his blood, which is love incorruptible" (Letter to the Romans 7:3 [A.D. 110]).

Let only that Eucharist be regarded as legitimate, which is celebrated under [the presidency of] the bishop or him to whom he has entrusted it (A.D.110).

Origen
"Formerly there was baptism in an obscure way . . . now, however, in full view, there is regeneration in water and in the Holy Spirit. Formerly, in an obscure way, there was manna for food; now, however, in full view, there is the true food, the flesh of the Word of God, as he himself says: ‘My flesh is true food, and my blood is true drink’ [John 6:55]" (Homilies on Numbers 7:2 [A.D. 248]).

Justin Martyr
"We call this food Eucharist, and no one else is permitted to partake of it, except one who believes our teaching to be true and who has been washed in the washing which is for the remission of sins and for regeneration [i.e., has received baptism] and is thereby living as Christ enjoined. For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by him, and by the change of which our blood and flesh is nurtured, is both the flesh and the blood of that incarnated Jesus" (First Apology 66 [A.D. 151]).

  What are the effects?

Irenaeus
"He has declared the cup, a part of creation, to be his own blood, from which he causes our blood to flow; and the bread, a part of creation, he has established as his own body, from which he gives increase unto our bodies. When, therefore, the mixed cup [wine and water] and the baked bread receives the Word of God and becomes the Eucharist, the body of Christ, and from these the substance of our flesh is increased and supported, how can they say that the flesh is not capable of receiving the gift of God, which is eternal life—flesh which is nourished by the body and blood of the Lord, and is in fact a member of him?" (ibid., 5:2). [A.D. 189]).
 
Clement of Alexandria
"’Eat my flesh,’ [Jesus] says, ‘and drink my blood.’ The Lord supplies us with these intimate nutrients, he delivers over his flesh and pours out his blood, and nothing is lacking for the growth of his children" (The Instructor of Children 1:6:43:3 [A.D. 191]).

Tertullian
The flesh, indeed, is washed [in baptism], in order that the soul may be cleansed . . . the flesh is shadowed with the imposition of hands [in confirmation], that the soul also may be illuminated by the Spirit; the flesh feeds [in the Eucharist] on the body and blood of Christ, that the soul likewise may be filled with God" (The Resurrection of the Dead 8 [A.D. 210]).