Is the Eastern Orthodox Church part of the Catholic Church?

They are. The answer to which you refer (“Quick Questions,” August 1991, p. 29) spoke of Eastern-rite Catholics as part of the Catholic Church, not of the Eastern Orthodox as part of the Catholic Church. While these two groups share a similar liturgical and cultural tradition, they’re distinct.

Eastern-rite Catholics are part of the Catholic Church, despite differences in custom and liturgical practice from Western Catholics. While Eastern Orthodox Christians have much in common with Catholics, they’re still in schism–they’ve split off from the legitimate authority of the pope–and therefore aren’t Catholics.

Overcoming the rift between the Catholic Church and Eastern Orthodoxy is at the top of Pope John Paul II’s ecumenical agenda. Mistakes were made on both sides, so we should pray for the Holy Spirit’s aid in healing old wounds and restoring full communion between the Eastern Orthodox churches and the Catholic Church.