Like my old geometry teacher used to say: "Some things are given..."
Origen
"If someone from this people wants to be saved, let him come into this house so that he may be able to attain his salvation. . . . Let no one, then, be persuaded otherwise, nor let anyone deceive himself: Outside of this house, that is, outside of the Church, no one is saved; for, if anyone should go out of it, he is guilty of his own death" (Homilies on Joshua 3:5 [A.D. 250]).
Cyprian of Carthage
Whoever is separated from the Church and is joined to an adulteress [a schismatic church] is separated from the promises of the Church, nor will he that forsakes the Church of Christ attain to the rewards of Christ. He is an alien, a worldling, and an enemy. He cannot have God for his Father who has not the Church for his mother" (The Unity of the Catholic Church 6, 1st ed. [A.D. 251]).
"Let them not think that the way of life or salvation exists for them, if they have refused to obey the bishops and priests, since the Lord says in the book of Deuteronomy: ‘And any man who has the insolence to refuse to listen to the priest or judge, whoever he may be in those days, that man shall die’ [Deut. 17:12]. And then, indeed, they were killed with the sword . . . but now the proud and insolent are killed with the sword of the Spirit, when they are cast out from the Church. For they cannot live outside, since there is only one house of God, and there can be no salvation for anyone except in the Church" (Letters 61[4]:4 [A.D. 253]).
Lactantius
"It is, therefore, the Catholic Church alone which retains true worship. This is the fountain of truth; this, the domicile of faith; this, the temple of God. Whoever does not enter there or whoever does not go out from there, he is a stranger to the hope of life and salvation. . . . Because, however, all the various groups of heretics are confident that they are the Christians and think that theirs is the Catholic Church, let it be known that this is the true Church, in which there is confession and penance and which takes a health-promoting care of the sins and wounds to which the weak flesh is subject" (Divine Institutes 4:30:11–13 [A.D. 307]).
Jerome
"Heretics bring sentence upon themselves since they by their own choice withdraw from the Church, a withdrawal which, since they are aware of it, constitutes damnation. Between heresy and schism there is this difference: that heresy involves perverse doctrine, while schism separates one from the Church on account of disagreement with the bishop. Nevertheless, there is no schism which does not trump up a heresy to justify its departure from the Church" (Commentary on Titus 3:10–11 [A.D. 386]).