Should Catholics venerate saints who were canonized by Eastern Orthodox churches?
It is possible for Catholics to admire the Christian witness of those recognized as saints by Eastern Orthodox churches. It is also possible for Catholics to learn from the lives and writings of apparently holy people of other Christian faith traditions. However, I do not recommend giving such people the honor Catholics accord to those who have been recognized to be saints by the Catholic Church.
Canonization by the pope is an infallible statement by the Church that a particular person definitely is in heaven and therefore is worthy to be given public honor by Catholics. Although the salvation of non-Catholics is hoped for, we have no such absolute knowledge that those “canonized” by non-Catholic churches indeed are in heaven.