Libanius tells us that, as Julian the Apostate marched to his Persian campaign, he spent one night at Pessinus, the home of the cult of Cybele and Attis, and wrote an essay defending the cult and interpreting its myth in philosophical style.
His oration on the Magna Mater, Oratio 5, is online here. It’s mostly full of allegorising, but begins with a valuable account of the coming of the cult of Cybele to Rome.
I’ve tried to condense a large chunk of the material to the bits which give concrete information about Attis, and paragraphed it for readability:
Who then is the Mother of the Gods? She is the Source of the Intelligible and Creative Powers, which direct the visible ones; she that gave birth to and copulated with the mighty Jupiter: she that exists as a great goddess next to the Great One, and in union with the Great Creator; she that is dispenser of all life; cause of all birth; most easily accomplishing all that is made; generating without passion; creating all that exists in concert with the Father; herself a virgin, without mother, sharing the throne of Jupiter, the mother in very truth of all the gods; for by receiving within herself the causes of all the intelligible deities that be above the world, she became the source to things the objects of intellect.
Now this goddess, who is also the same as Providence, was seized with a love without passion for Attis. … And this the legend aims at teaching when it makes the Mother of the Gods enjoin upon Attis to be her servant, and not to stray from her, and not fall in love with another woman. But he went forward, and descended as far as the boundaries of Matter.
But when it became necessary for this ignorance to cease and be stopped—-then Corybas, the mighty Sun, the colleague of the Mother of the Gods … persuades the lion to turn informer. Who then is this lion? We hear him styled “blazing”—-he must, therefore, I think, be the cause presiding over the hot and fiery element; that which was about to wage war against the Nymph, and to make her jealous of her intercourse with Attis; and who this Nymph is we have already stated.
This lion, the fable tells, lent his aid to the Mother of the Gods … and by his detecting the offence and turning informer, became the author of the castration of the youth. … not without the intervention of the fabled madness of Attis…
It is not therefore unreasonable to suppose this Attis a sixper-natural personage (in fact the fable implies as much), or rather in all respects, a deity, seeing that he comes forth out of the Third Creator, and returns again after his castration, to the Mother of the Gods… the fable styles him a “demi-god,” … The Corybantes… are assigned by the Great Mother to act as his bodyguard…
This great god of ours is Attis; this is the meaning of the “Flight of King Attis” that we have just been lamenting; his “Concealments,” his “Vanishings,” his “Descents into the Cave.” Let my evidence be the time of year when all these ceremonies take place; for it is said that the Sacred Tree is cut down at the moment when the Sun arrives at the extreme point of the equinoctial arc: next in order follows the Sounding of the trumpets, and lastly is cut down the sacred and ineffable Harvest of the god Gallos: after these come, as they say, the Hilaria and festivities.
Now that a “cessation of Indefinity” is meant by the castration so much talked of by the vulgar, is self-evident from the fact that when the Sun touches the equinoctial circle, where that which is most definite is placed (for equality is definite, but inequality indefinite and inexplicable); at that very moment (according to the report), the Sacred Tree is cut down; then come the other rites in their order; whereof some are done in compliance with rules that be holy and not to be divulged; others for reasons allowable to be discussed.
The “Cutting of the Tree;” this part refers to the legend about the Gallos, and has nothing to do with the rites which it accompanies… The rite, therefore, enjoins upon us who are celestial by our nature, but who have been carried down to earth, to reap virtue joined with piety from our conduct upon earth, and to aspire upwards unto the deity, the primal source of being and the fount of life. Then immediately after the cutting does the trumpet give out the invocation to Attis and to those that be of heaven, whence we took our flight, and fell down to earth.
And after this, when King Attis checks the Indefinity by the means of castration, the gods thereby warn us to extirpate in ourselves all incontinence, and to imitate the example, and to run upwards unto the Definite, and the Uniform, and if it be possible, to the One itself; which being accomplished the “Hilaria” must by all means follow. For what could be more contented, what more hilarious than the soul that has escaped from uncertainty, and generation, and the tumult that reigns therein, and hastens upwards to the gods? Of whose number was this Attis, whom the Mother of the Gods would not suffer to advance farther than was proper for him, but turned him towards herself, and enjoined him to check all indefinity.