"The Lord says to Peter: ‘I say to you,’ he says, ‘that you are Peter, and upon this rock I will build my Church.’ . . . On him [Peter] he builds the Church, and to him he gives the command to feed the sheep [John 21:17], and although he assigns a like power to all the apostles, yet he founded a single chair [cathedra], and he established by his own authority a source and an intrinsic reason for that unity. Indeed, the others were that also which Peter was [i.e., apostles], but a primacy is given to Peter, whereby it is made clear that there is but one Church and one chair. So too, all [the apostles] are shepherds, and the flock is shown to be one, fed by all the apostles in single-minded accord. If someone does not hold fast to this unity of Peter, can he imagine that he still holds the faith? If he [should] desert the chair of Peter upon whom the Church was built, can he still be confident that he is in the Church?" (The Unity of the Catholic Church 4; 1st edition [A.D. 251]).—St Cyprian of Carthage
The division among non-Catholic communities is a pitiable sign of discord and confusion so far from Jesus' express will for His Church that Catholics must pray constantly for those people of goodwill who are lost to heresy and schism. THey so desperately need the unity (in the Truth) which only the Petrine ministry can provide.
We Catholics must be mindful, too, of the signs of God's grace among those same people of goodwill who currently do not enjoy the fullness of the Faith in unity with Jesus' earthly shepherd and vicar, the Roman Pontiff, the Bishop of Rome.
(M)any elements of sanctification and of truth" are found outside the visible confines of the Catholic Church: "the written Word of God; the life of grace; faith, hope, and charity, with the other interior gifts of the Holy Spirit, as well as visible elements." Christ's Spirit uses these Churches and ecclesial communities as means of salvation, whose power derives from the fullness of grace and truth that Christ has entrusted to the Catholic Church. All these blessings come from Christ and lead to him, and are in themselves calls to "Catholic unity."—CCC819