St. Ignatius of Antioch, disciple of John, Epistle to the Smyrnaeans, 110 A.D.
"6:1 Let no man be deceived. Even the heavenly things, and the glory of the angels, and the principalities, both visible and invisible, if they believe not on the blood of Christ, for them also is there condemnation. Let him who receiveth it, receive it in reality. Let not high place puff up any man. For the whole matter is faith and love, to which there is nothing preferable.
6:2 Consider those who hold heretical opinions with regard to the grace of Jesus Christ which hath come unto us, how opposite they are to the mind of God. They have no care for love, nor concerning the widow, nor concerning the orphan, nor concerning the afflicted, nor concerning him who is bound or loosed, nor concerning him who is hungry or thirsty. They refrain from the eucharist and from prayer, because they do not confess that the eucharist is the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father of his goodness raised up.
7:1 They, therefore, who speak against the gift of God, die disputing. But it were better for them to love, that they might also rise again.
7:2 It is, therefore, proper to abstain from such, and not to speak concerning them, either in private or in public; but to attend to the prophets, and especially to the gospel, in which the passion hath been revealed unto us, and the resurrection hath been perfected.
8:1 But avoid divisions, as being the beginning of evils. Do ye all follow the bishop, as Jesus Christ doth the Father; and follow the presbyters as the apostles; and have respect unto the deacons as unto the commandment of God. Let no one, apart from the bishop, do any of the things that appertain unto the church. Let that eucharist alone be considered valid which is celebrated in the presence of the bishop, or of him to whom he shall have entrusted it.
8:2 Wherever the bishop appear, there let the multitude be; even as wherever Christ Jesus is, there is the Catholic Church. It is not lawful either to baptize, or to hold a love-feast without the consent of the bishop; but whatsoever he shall approve of, that also is well pleasing unto God, to the end that whatever is done may be safe and sure."
St. Justin Martyr, First Apology, c. 150 A.D.
"And this food is called among us Eucharist, of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body;" and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood;" and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn."