17 And even among all the communities of Oriental Christians which long since have broken away from the unity of the Catholic Church, there were not wanting, nor are there wanting, those who, although animated by prejudices and wrong opinions, have embraced this doctrine and celebrate annually the solemnity of the Immaculate Conception; which would undoubtedly not be so, had they not received this doctrine from ancient times, before they were cut off from the one fold.
Source: Fulgens Corona (Vatican.va)