Are Eastern Orthodox Canonizations Valid?

There are several saints in the offical Roman rite martyrology published under St. John Paul II who lived after the Great Schism of East and West—for example, Sts. Boris and Gleb and St. Sergius of Radonezh in Russia. Some of the Byzantine rite usages, for example, venerate St. Gregory Palamas, whose office is in an appendix in the office book published for Greek Catholics in Italy. So the simple answer is yes, the Catholic Church would generally accept the veneration of Orthodox saints in the case where there is a reconciliation of the communities. This seems to be the case with St. Seraphim of Sarov for Russian Catholics under St. Pius X.

A lot of this is just a question of local custom. This does not mean the Catholic Church would approve of all the actions or teaching of these saints, any more than it does for its own officially canonized saints. It is clear, however, that this attitude is not reciprocated by the Orthodox for Catholic saints. An example of this is the patriarch of Constantinople’s Orthodox canonization of St. Stephen of Hungary just a few years ago; St. Stephen was canonized already in the twelfth century!