Old encyclicals, simple and clear, without any detriment to the truth, combined the words about schism and brotherhood. This is why we love previous popes writing on heretics and schismatics with a truly fatherly care and love.
„For the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it. To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it.
Let, therefore, the separated children draw nigh to the Apostolic See, set up in the City which Peter and Paul, the Princes of the Apostles, consecrated by their blood; to that See, We repeat, which is »the root and womb whence the Church of God springs«, not with the intention and the hope that »the Church of the living God, the pillar and ground of the truth« will cast aside the integrity of the faith and tolerate their errors, but, on the contrary, that they themselves submit to its teaching and government. Would that it were Our happy lot to do that which so many of Our predecessors could not, to embrace with fatherly affection those children, whose unhappy separation from Us We now bewail. Would that God our Saviour, “Who will have all men to be saved and to come to the knowledge of the truth,” would hear us when We humbly beg that He would deign to recall all who stray to the unity of the Church! In this most important undertaking We ask and wish that others should ask the prayers of Blessed Mary the Virgin, Mother of divine grace, victorious over all heresies and Help of Christians, that She may implore for Us the speedy coming of the much hoped-for day, when all men shall hear the voice of Her divine Son, and shall be »careful to keep the unity of the Spirit in the bond of peace«.
You, Venerable Brethren, understand how much this question is in Our mind, and We desire that Our children should also know, not only those who belong to the Catholic community, but also those who are separated from Us: if these latter humbly beg light from heaven, there is no doubt but that they will recognize the one true Church of Jesus Christ and will, at last, enter it, being united with us in perfect charity.”
Fragments of the encyclical of the pope whom we cherish, Pius XI, Mortalium animos.