Then there arose differences in dogma; and Rome considered the Church in the East schismatic, and Byzantium held that that of the West was heterodox.

They now not only disapproved of each other's methods, but what was more serious, held different creeds.
The Latin Church, after its Bishop had become an infallible Pope (about the middle of the fifth century), claimed that the Church in the East must accept his definition of dogma as final.
It was one small word which finally rent these two bodies of Christendom forever apart.

It was only the word _filioque_ which made the impassable gulf dividing them.

The Latins maintained that the Holy Spirit proceeded from the Father--_and the son_; the Greeks that it descended from the Father alone.

It was the undying controversy concerning the relations and the attributes of the three Members of the Trinity; and the insoluble question was destined to break up Greek and Catholic Church alike into numberless sects and shades of belief or unbelief; and over this Christological controversy, rivers of blood were to flow in both branches of Christendom.
The theological question involved was of course too subtle for ordinary comprehension.