Let no man deceive himself: if anyone be not within the altar, he is deprived of the bread of God. For if the prayer of one or two possesses (Matt. 18:19) such power, how much more that of the bishop and the whole Church! He, therefore, that does not assemble with the Church, has even by this manifested his pride, and condemned himself. (Ignatius, To the Ephesians 5)
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. (Ignatius, To the Philadelphians 3)
And as in the sea there are islands, some of them habitable, and well-watered, and fruitful, with havens and harbors in which the storm-tossed may find refuge,—so God has given to the world which is driven and tempest-tossed by sins, assemblies—we mean holy churches—in which survive the doctrines of the truth, as in the island-harbors of good anchorage; and into these run those who desire to be saved, being lovers of the truth, and wishing to escape the wrath and judgment of God. And as, again, there are other islands, rocky and without water, and barren, and infested by wild beasts, and uninhabitable, and serving only to injure navigators and the storm-tossed, on which ships are wrecked, and those driven among them perish,—so there are doctrines of error—I mean heresies—which destroy those who approach them. For they are not guided by the word of truth; but as pirates, when they have filled their vessels, drive them on the fore-mentioned places, that they may spoil them: so also it happens in the case of those who err from the truth, that they are all totally ruined by their error. (Theophilus, To Autolycus II, 14)
The spouse of Christ cannot be adulterous; she is uncorrupted and pure. She knows one home; she guards with chaste modesty the sanctity of one couch. She keeps us for God. She appoints the sons whom she has born for the kingdom. Whoever is separated from the Church and is joined to an adulteress, is separated from the promises of the Church; nor can he who forsakes the Church of Christ attain to the rewards of Christ. He is a stranger; he is profane; he is an enemy. He can no longer have God for his Father, who has not the Church for his mother. If anyone could escape who was outside the ark of Noah, then he also may escape who shall be outside of the Church. (Cyprian, On the Unity of the Church 6)
But, moreover, the very interrogation which is put in baptism is a witness of the truth. For when we say, “Dost thou believe in eternal life and remission of sins through the holy Church?” we mean that remission of sins is not granted except in the Church, and that among heretics, where there is no Church, sins cannot be put away. (Cyprian, Epistle 69.2)
Moreover, all other heretics, if they have separated themselves from the Church of God, can have nothing of power or of grace, since all power and grace are established in the Church where the elders453 preside, who possess the power both of baptizing, and of imposition of hands, and of ordaining. (Cyprian, Epistle 74.7)
But what is the greatness of his error, and what the depth of his blindness, who says that remission of sins can be granted in the synagogues of heretics, and does not abide on the foundation of the one Church which was once based by Christ upon the rock, may be perceived from this, that Christ said to Peter alone, “Whatsoever thou shall bind on earth shall be bound in heaven, and whatsoever thou shall loose on earth shall be loosed in heaven.” (Matt. 16:19) (Cyprian, Epistle 74.16)
Obviously, the fathers have quite a bit more in mind than simply “going to church.” But none of what they have in mind happens without a person first going to church…faithfully.