Heretics do not accept the Eucharist as Real Presence

Do ye, therefore, notice those who preach other doctrines, how they affirm that the Father of Christ cannot be known, and how they exhibit enmity and deceit in their dealings with one another. They have no regard for love; they despise the good things we expect hereafter; they regard present things as if they were durable; they ridicule him that is in affliction; they laugh at him that is in bonds.

They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Savior Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again.

Ignatius of Antioch
Epistle to the Smyrneans
Chapter VI-VII

One Eucharist

Take ye heed, then, to have but one Eucharist. For there is one flesh of our Lord Jesus Christ, and one cup to show forth the unity of His blood; one altar; as there is one bishop, along with the presbytery and deacons, my fellow-servants: that so, whatsoever ye do, ye may do it according to the will of God. I have confidence of you in the Lord, that ye

Ignatius of Antioch
Epistle to the Philadelphians
Chapter IV

Avoid schisms. Remain with the bishop.

For as many as are of God and of Jesus Christ are also with the bishop. And as many as shall, in the exercise of repentance, return into the unity of the Church, these, too, shall belong to God, that they may live according to Jesus Christ. Do not err, my brethren. If any man follows him that makes a schism in the Church, he shall not inherit the kingdom of God. If any one walks according to a strange opinion, he agrees not with the passion of Christ.

Ignatius of Antioch
Epistle to the Philadelphians
Chapter III

I desire the flesh and blood of Christ

I have no delight in corruptible food, nor in the pleasures of this life. I desire the bread of God, the heavenly bread, the bread of life, which is the flesh of Jesus Christ, the Son of God, who became afterwards of the seed of David and Abraham; and I desire the drink of God, namely His blood, which is incorruptible love and eternal life.

Ignatius of Antioch
Epistle to the Romans
Chapter VII

According to the love of Jesus Christ our God

Ignatius, who is also called Theophorus, to the Church which has obtained mercy, through the majesty of the Most High Father, and Jesus Christ, His only-begotten Son; the Church which is beloved and enlightened by the will of Him that wills all things which are according to the love of Jesus Christ our God, which also presides in the place of the region of the Romans, worthy of God, worthy of honor, worthy of the highest happiness, worthy of praise, worthy of obtaining her every desire, worthy of being deemed holy,  and which presides over love, is named from Christ, and from the Father, which I also salute in the name of Jesus Christ, the Son of the Father: to those who are united, both according to the flesh and spirit, to every one of His commandments; who are filled inseparably with the grace of God, and are purified from every strange taint, I wish abundance of happiness unblameably, in Jesus Christ our God.

Ignatius of Antioch
Epistle to the Romans
Chapter I