I have already remarked on the fact that Pope Francis made a non-Catholic a Doctor of the Catholic Church. St. Gregory Narek was a monk in the Armenian Apostolic Church, who died in the early 11th century.
I also wrote a post on how Eastern Catholic Churches have brought Orthodox saints with them into the Catholic Church. This includes figures like the Patriarch Photius, who strongly criticized the Papacy and Catholic Trinitarian theology (although who did die in communion with Rome). Occasionally, it has included embracing Orthodox saints from after the time of reunion, such as Churches in the Slavonic tradition celebrating St. Seraphim of Sarov (1754-1833), a Russian monk who was included in the revision of the martyrology by John Paul II.
Pope Francis has done something similar, although with no liturgical precedent (no prior liturgical celebration by Catholics), with the 21 Coptic Martyrs, martyred in Libya in 2015. He stated:
I am glad to announce today that, with the consent of Your Holiness, these 21 martyrs will be inserted into the Roman Martyrology as a sign of the spiritual communion uniting our two Churches. . . . May the prayer of the Coptic martyrs, united with that of the Theotokos, continue to grow the friendship between our Churches, until the blessed day when we can celebrate together at the same altar and commune in the same Body and Blood of the Savior, ‘that the world may believe.’
Catholic News Agency, 5/11/23
Pope Tawadros II, head of Coptic Orthodox Church, gifted relics of the martyr to the Pope. He stated:
Today we hand over part of their relics, dipped in their blood shed in the name of Christ for the Church, so that they may be remembered in the martyrology of all the Churches of the world, and know ‘we too’ are ‘surrounded by such a multitude of witnesses. . . . Precisely because the saints are one of the main pillars of our Churches, beginning with the apostles Peter, Paul, and Mark . . . we now write in the martyrology of the Churches the new martyrs who have guarded the faith and bore witness to Christ, who did not lose heart in the face of torture and passed on to us a living example in martyrdom.
Catholic News Agency, 5/11/23
This communion through the saints points us to a unique ecumenism, recognizing and celebrating holiness in Churches that maintain the apostolic tradition. It seems to point us toward growing unity, hopefully anticipating sacramental communion. Perhaps the saints will be the ones who will bring us to the unity we have not been able to achieve on our own.
On the Coptic martyrs, I highly recommend Martin Mosebach’s book, The 21: A Journey into the Land of the Coptic Martyrs. It opens up the world of Coptic Christians in Egypt through the eyes of a Catholic visitor.