I wasn’t able to find this text through a Google search. Since you quoted it, perhaps you can find it in context for us?
I found it in the Verbum Catholic Document Library.
The Necessity of Union with the Church
[From epistle (2) “Dilectionis vestrae” to the schismatic bishops of Istria, about 585]
247 [DS 468] … Do not (therefore) because of a love of ostentation, which is always next to pride, remain in the vice of obstinacy; since in the day of judgment no one can excuse himself.…
For although it is evident from the word of the Lord Himself in the Sacred Gospel [cf. Matt. 16:18] where the Church is established, let us hear nevertheless what the blessed Augustine, mindful of the opinion of the same Lord, has explained. For he says that the Church of God is established among those who are known to preside over the apostolic sees, through the succession of those in charge, and whoever separates himself from the communion or authority of these sees, is shown to be in schism. And following additional remarks (he says): “If you are put outside, for the name of Christ you will also die. Suffer for Christ among the members of Christ; clinging to the body, fight for the head.” [DS 469] But the blessed Cyprian … among other things, says the following: “The beginning starts from unity, and the primacy is given to PETER, so that the Church and the chair of Christ may be shown (to be) one: and they are all shepherds, but the flock, which is fed by the Apostles in unanimous agreement, is shown to be one.” And after a few (remarks he adds): “Does he who does not hold this unity of the Church believe that he has the faith? Does he who deserts and resists the chair of PETER, on which the Church was founded, have confidence that he is in the Church?” Likewise after other remarks (he asserts): “They cannot arrive at the reward of peace, because they disrupt the peace of the Lord by the fury of discord.… Those who were not willing to be at agreement in the Church of God, cannot remain with God; although given over to flames and fires, they burn, or thrown to wild beasts, they lay down their lives, there will not be [for them] that crown of faith, but the punishment of faithlessness, not a glorious result (of religious virtue), but the ruin of despair. Such a one can be slain, he cannot be crowned.… For the crime of schism is worse than that which they [commit] who have offered sacrifice, who, nevertheless, having been disposed to penance for their sins prayed to God with the fullest satisfaction. In this case the Church is sought and solicited; in the other the Church is opposed. So in this case he who has fallen, has injured only himself; in the other, who attempts to cause a schism deceives many by dragging (them) with himself. In this case there is the loss of one soul; in the other there is danger to many. Certainly the one knows that he has sinned and laments and bewails (it); the other puffed up with pride in his sin and pluming himself on the sins themselves, separates sons from their mother, seduces the sheep from the shepherds, disturbs the sacraments of God, and, whereas the former having stumbled sinned once, the latter sins daily. Lastly although the lapsed, if afterwards he acquired martyrdom, is able to secure the promises of the kingdom; if the other is slain outside of the Church, he cannot attain to the rewards of the Church.”