These two stories from On the Lapsed relate accounts of those who had lapsed during the Decian persecution attempting to receive Holy Communion before being properly reconciled to the Church:
"[A] woman who in advanced life and of more mature age secretly crept in among us when we were sacrificing, received not food, but a sword for herself; and as if taking some deadly poison into her jaws and body, began presently to be tortured, and to become stiffened with frenzy; and suffering the misery no longer of persecution, but of her crime, shivering and trembling, she fell down. The crime of her dissimulated conscience was not long unpunished or concealed. She who had deceived man felt that God was taking vengeance" (On the Lapsed, 26).
"[W]hen one, who himself was defiled, dared with the rest to receive secretly a part of the sacrifice celebrated by the priest; he could not eat nor handle the holy of the Lord, but found in his hands when opened that he had a cinder" (ibid.)
Here Cyprian reflects on the connection between being in union with Christ through the sacrament of His Body and being in union with the Church, which is His mystical body:
"Christ is the bread of those who are in union with His body. And we ask that this bread should be given to us daily, that we who are in Christ, and daily receive the Eucharist for the food of salvation, may not, by the interposition of some heinous sin, by being prevented, as withheld and not communicating, from partaking of the heavenly bread, be separated from Christ's body, as He Himself predicts, and warns, "I am the bread of life which came down from heaven. If any man eat of my bread, he shall live for ever: and the bread which I will give is my flesh, for the life of the world." When, therefore, He says, that whoever shall eat of His bread shall live for ever; as it is manifest that those who partake of His body and receive the Eucharist by the right of communion are living, so, on the other hand, we must fear and pray lest any one who, being withheld from communion, is separate from Christ's body should remain at a distance from salvation; as He Himself threatens, and says, "Unless you eat the flesh of the Son of man, and drink His blood, you shall have no life in you." And therefore we ask that our bread— that is, Christ— may be given to us daily, that we who abide and live in Christ may not depart from His sanctification and body" (On the Lord's Prayer, 18).
Here are some more quotes by the good Bishop of Carthage on the Eucharist; note the connection between the the blood of Christ being consumed as preparation for the martyr shedding his own blood for Christ:
"But now indeed peace is necessary, not for the sick, but for the strong; nor is communion to he granted by us to the dying, but to the living, that we may not leave those whom we stir up and exhort to the battle unarmed and naked, but may fortify them with the protection of Christ's body and blood. And, as the Eucharist is appointed for this very purpose that it may be a safeguard to the receivers, it is needful that we may arm those whom we wish to be safe against the adversary with the protection of the Lord's abundance. For how do we teach or provoke them to shed their blood in confession of His name if we deny to those who are about to enter on the warfare the blood of Christ? Or how do we make them fit for the cup of martyrdom, if we do not first admit them to drink, in the Church, the cup of the Lord by the right of communion?" (Letter 53:2)"[T]he soldiers of Christ ought to prepare themselves with uncorrupted faith and robust courage, considering that they drink the cup of Christ's blood daily, for the reason that they themselves also may be able to shed their blood for Christ" (Letter 55:1)."Let us also arm the right hand with the sword of the Spirit, that it may bravely reject the deadly sacrifices; that, mindful of the Eucharist, the hand which has received the Lord's body may embrace the Lord Himself, hereafter to receive from the Lord the reward of heavenly crowns" (Letter 55:9).
Though a more precise sacramental theology would not arise until the later Middle Ages, Cyprian makes some early contributions to the ideas of matter and form when he addresses the problem of certain Churches attempting to consecrate the Eucharist with water instead of wine. Note his statement that the blood cannot "appear" to be in the cup unless the proper form (wine) is present:
"For when Christ says, "I am the true vine", the blood of Christ is assuredly not water, but wine; neither can His blood by which we are redeemed and quickened appear to be in the cup, when in the cup there is no wine whereby the blood of Christ is shown forth, which is declared by the sacrament and testimony of all the Scriptures" (Letter 62:2).
St. Cyprian frequently refers to the offering of the Eucharist as a sacrifice and calls the Mass "the sacrifices of God." For example:
"Yea, it is the great honor and glory of our episcopate to have granted peace to martyrs, so that we, as priests, who daily celebrate the sacrifices of God, may prepare offerings and victims for God" (Letter 52:3).
"[W]e ought in the ordinations of priests to choose none but unstained and upright ministers, who, holily and worthily offering sacrifices to God, may be heard in the prayers which they make for the safety of the Lord's people" (Letter 67:2).
Finally, on the wonderful effects of receiving the sacrament of the Lord's Body and Blood:
"When the blood of the Lord and the cup of salvation have been drunk, the memory of the old man is laid aside, and there arises an oblivion of the former worldly conversation, and the sorrowful and sad breast which before was oppressed by tormenting sins is eased by the joy of the divine mercy; because that only is able to rejoice him who drinks in the Church which, when it is drunk, retains the Lord's truth" (Letter 62:11).
St. Cyprian, ora pro nobis!