“…this is common both to the Greeks and to the barbarians, to perform their sacred rites in connection with the relaxation of a festival… And it is in accordance with the dictates of nature that this should be so, for,
- in the first place, the relaxation draws the mind away from human occupations and turns the real mind towards that which is divine; and,
- secondly, the religious frenzy seems to afford a kind of divine inspiration and to be very like that of the soothsayer; and,
- thirdly, the secrecy with which the sacred rites are concealed induces reverence for the divine, since it imitates the nature of the divine, which is to avoid being perceived by our human senses; and,
- fourthly, music, which includes dancing as well as rhythm and melody, at the same time, by the delight it affords and by its artistic beauty, brings us in touch with the divine, and this for the following reason;
for although it has been well said that human beings then act most like the [Goddesses and G]ods when they are doing good to others, yet one might better say, when they are happy; and such happiness consists of rejoicing, celebrating festivals, pursuing philosophy, and engaging in music…”
http://www.perseus.tufts.edu/hopper/text?doc=Strab.+10.3.9&fromdoc=Perseus%3Atext%3A1999.01.0198
Strabo 10.3.10 on music and religion:
“…the Pythagoreans, called philosophy music;196 and they say that the universe is constituted in accordance with harmony,197 assuming that every form of music is the work of the [Goddesses and G]ods.
And in this sense, also, the Muses are [G]oddesses, and Apollo is leader of the Muses, and poetry as a whole is laudatory of the [Goddesses and G]ods.
And by the same course of reasoning they also attribute to music the upbuilding of morals, believing that everything which tends to correct the mind is close to the [Goddesses and G]ods.
Now most of the Greeks assigned to Dionysus, Apollo, Hecate, the Muses, and above all to Demeter, everything of an orgiastic or Bacchic or choral nature, as well as the mystic element in initiations;
and they give the name "Iacchus" not only to Dionysus but also to the leader-in-chief of the mysteries, who is the genius of Demeter.
And branch-bearing, choral dancing, and initiations are common elements in the worship of these [Goddesses and G]ods.
As for the Muses and Apollo, the Muses preside over the choruses, whereas Apollo presides both over these and the rites of divination.
But all educated men (sic), and especially the musicians, are ministers of the Muses;
and both these and those who have to do with divination are ministers of Apollo;
and the initiated and torch-bearers and hierophants, of Demeter;
and the Sileni and Satyri and Bacchae, and also the Lenae and Thyiae and Mimallones and Naïdes and Nymphae and the beings called Tityri, of Dionysus.”
http://www.perseus.tufts.edu/hopper/text?doc=Strab.+10.3.10&fromdoc=Perseus%3Atext%3A1999.01.0198