Pope Leo IX excommunicated Patriarch Michael Cerularius of
Constantinople, who, in turn, excommunicated Pope Leo in 1054 AD.
This was caused because of disagreements over the unauthorized
changes made by Pope Leo to the Nicene Creed (by adding the
"Filioque clause"). The Orthodox Church maintains that Holy Spirit
"proceeds from the Father alone, whereas the Roman Catholics
claimed that the Holy Spirit proceeds from the Father "and the Son"
which goes against the Bible. "The Bible states that ...the Spirit
of truth who proceeds from the Father..." (John 15:26).
However, both of these excommunications were mutually removed in
1965 by Pope Paul VI and Patriarch Athenagoras I.