St. Ignatius of Antioch: . . . if anyone follow a schismatic . . .

Icon of St. Ignatius of Antioch“Do not err, my brethren: if anyone follow a schismatic, he will not inherit the Kingdom of God. If any man walk about with strange doctrine, he cannot lie down with the passion. Take care, then, to use one Eucharist, so that whatever you do, you do according to God: for there is one Flesh of our Lord Jesus Christ, and one cup in the union of His Blood; one altar, as there is one bishop with the presbytery and my fellow servants, the deacons.”

— St. Ignatius Of Antioch, Epistle to the Philadelphians, 3:2-4:1, 110 A.D.