Father;" to this phrase the words, "and the Son," had been added in the West, originally by some Spanish bishops; the Greeks protested against an unauthorised addition being inserted into a creed promulgated by a general council, and received by the universal Church as the symbol of faith. Thus arose the celebrated controversy on the "Filioque," which was one of the chief causes of the great schism between the Eastern and Western Churches in the ninth century. The Arian, Manichæan, Marcionite, and Monothelite heresies spread, during this century, through the Greek Church, and, where the Arabians ruled, the Nestorians and Monophysites also flourished. In the Latin Church a phase of the Nestorian heresy made its way, under the name of Adoptianism, a name given because its adherents regarded Christ, so far as his manhood was concerned, as the Son of God by adoption only. CENTURY IX. Christendom, during this century, as during the preceding one, was threatened and harassed by the inroads of Mahommedan powers, and the first gleams of returning light began to penetrate its thick darkness--light proceeding from the Arabians and the Saracens, the restorers of knowledge and of science. It is not here our duty to trace that marvellous work of the revival of thought--thought which Christianity had slain, but which, revived by Mahommedanism, was destined to issue in the new birth of heretic philosophy. While this work was proceeding among the Saracens, the Arabians, and the Moors, Christendom went on its way, degraded, vicious, and superstitious; only here and there an effort at learning was made, and some few went to the Arabian schools, and returned with some tincture of knowledge. John Scotus Erigena, a subtle and acute thinker, left behind him works which have made some regard him as the founder of the _Realist_ school of the middle ages, the school which followed Aristotle, in opposition to the _Nominalists_, who held with Zeno and the Stoics. Erigena taught that the soul would be re-absorbed into the divine spirit, from which it had originally emanated; from God all things had come--to Him would they ultimately return; God alone was eternal, and in the end nothing but God would exist. Some of Erigena's works naturally fell under the displeasure of the Church, and were duly burned: he was a philosopher, and therefore dangerous.