In his twenty-eighth year he [Plotinus] felt the impulse to study philosophy and was recommended to the teachers in Alexandria who then had the highest reputation; but he came away from their lectures so depressed and full of sadness that he told his trouble to one of his friends. The friend, understanding the desire of his heart, sent him to Ammonius, whom he had not so far tried. He went and heard him, and said to his friend, "This is the man I was looking for." From that day he stayed continually with Ammonius and acquired so complete a training in philosophy that he became eager to make acquaintance with the Persian philosophical discipline and that prevailing among the Indians.
According to Porphyry, the parents of Ammonius were Christians, but upon learning Greek philosophy, Ammonius rejected his parents' religion for paganism. This conversion is contested by the Christian writers Jerome and Eusebius, who state that Ammonius remained a Christian throughout his lifetime:
[Porphyry] plainly utters a falsehood (for what will not an opposer of Christians do?) when he says that ... Ammonius fell from a life of piety into heathen customs. ... Ammonius held the divine philosophy unshaken and unadulterated to the end of his life. His works yet extant show this, as he is celebrated among many for the writings which he has left.Eusebius goes on to mention a work On the Harmony of Moses and Jesus, and in an epistle addressed to Carpianus speaks of a Diatessaron or Harmony of the Four Gospels composed by Ammonius.
However we are told by Longinus that Ammonius wrote nothing, and if Ammonius was the principle influence on Plotinus, then it is unlikely that Ammonius would have been a Christian. One way to explain much of the confusion concerning Ammonius is to assume that there were two people called Ammonius: Ammonius Saccas who taught Plotinus, and an Ammonius the Christian who wrote biblical texts.
To add to the confusion, it seems that Ammonius had two pupils called Origen: Origen the Christian, and Origen the Pagan. It is quite possible that Ammonius Saccas taught both Origens. Among Ammonius' other pupils there were Herennius and Cassius Longinus.
He was the first who had a godly zeal for the truth in philosophy and despised the views of the majority, which were a disgrace to philosophy. He apprehended well the views of each of the two philosophers [Plato and Aristotle] and brought them under one and the same nous and transmitted philosophy without conflicts to all of his disciples, and especially to the best of those acquainted with him, Plotinus, Origen, and their successors.
Little is known about Ammonius's role in the development of Neoplatonism. Porphyry seems to suggest that Ammonius was instrumental in helping Plotinus think about philosophy in new ways:
But he [Plotinus] did not just speak straight out of these books but took a distinctive personal line in his consideration, and brought the mind of Ammonius' to bear on the investigation in hand.Two of Ammonius's students - Origen the Pagan, and Longinus - seem to have held philosophical positions which were closer to Middle Platonism than Neoplatonism, which perhaps suggests that Ammonius's doctrines were also closer to those of Middle Platonism than the Neoplatonism developed by Plotinus (see the Enneads), but Plotinus does not seem to have thought that he was departing in any significant way from that of his master.