Does the Holy Spirit proceed from just God the Father, as Orthodox doctrine has insisted since 381? Or from the Father and the Son, as the Vatican decreed in 1014, when it rewrote the Nicene Creed? (That change has divided the two Christian branches for nearly 1,000 years.)
"This is still under discussion by the Vatican," Schreck said, "but when Pope John Paul prays with the Orthodox, he prays as they do as a way of seeking to reconcile the two branches. The Western Catholic Church's view is influenced by St. Augustine's writing of the Holy Spirit as a mutual procession from the Father and the Son."
1.—Since there is a quarrel between the Romans and Greeks about the procession of the Holy Spirit, which greatly impede unity really for no other reason than that we do not wish to understand one another—we ask that we should not be compelled to any other creed but that we should remain with that which was handed down to us in the Holy Scriptures, in the Gospel, and in the writings of the holy Greek Doctors, that is, that the Holy Spirit proceeds, not from two sources and not by a double procession, but from one origin, from the Father through the Son. (Union of Brest)
But only the theology behind it, which is aptly expressed by St. Basil in his Adversus Eunomium:
Even if the Holy Spirit is third in dignity and order, why need he be third also in nature? For that he is second to the Son, having his being from him and receiving from him and announcing to us and being completely dependent on him, pious tradition recounts; but that his nature is third we are not taught by the Saints nor can we conclude logically from what has been said.
The Nicene creed written up by St Athanasius contained the filioque. Arian heretics who didn't believe that Jesus was fully God sought to remove the filioque, and "orthodox" heretics today are doing the same.