Now moderation, adequacy, excess in nothing, and complete self-sufficiency are above all else the essential characteristics of everything done by the gods; and if anyone should take this fact as a starting-point, and assert that Greece has far more than its share in the general depopulation which the earlier discords and wars have wrought throughout practically the whole inhabited earth, and that to-day the whole of Greece would hardly muster three thousand men-at-arms, which is the number that the one city of the Megarians sent forth to Plataeae (for the god’s abandoning of many oracles is nothing other than his way of substantiating the desolation of Greece), in this way such a man would give some accurate evidence of his keenness in reasoning. For who would profit if there were an oracle in Tegyrae, as there used to be, or at Ptoüm, where during some part of the day one might possibly meet a human being pasturing his flocks? And regarding the oracle here at Delphi, the most ancient in time and the most famous in repute, men record that for a long time it was made desolate and unapproachable by a fierce creature, a serpent; they do not, however, put the correct interpretation upon its lying idle, but quite the reverse; for it was the desolation that attracted the creature rather than that the creature caused the desolation. But when Greece, since God so willed, had grown strong in cities and the place was thronged with people, they used to employ two prophetic priestesses who were sent down in turn; and a third was appointed to be held in reserve. But to-day there is one priestess and we do not complain, for she meets every need. There is no reason, therefore, to blame the god; the exercise of the prophetic art which continues at the present day is sufficient for all, and sends away all with their desires fulfilled. Agammemnon, for example, used nine heralds and, even so, had difficulty in keeping the assembly in order because of the vast numbers; but here in Delphi, a few days hence, in the theatre you will see that one voice reaches all. In the same way, in those days, prophecy employed more voices to speak to more people, but to-day, quite the reverse, we should needs be surprised at the god if he allowed his prophecies to run to waste, like water, or to echo like the rocks with the voices of shepherds and flocks in waste places.
Plutarch, The Obsolescence of Oracles, 413F-414C
Translation by Frank Cole Babbitt, LCL 306
The above is the core of Plutarch’s explanation for the various oracles of the Greek world having fallen silent in the late first and early second centuries AD: there weren’t enough people for them to be necessary. In a way, he may be right, in that votive donations will have steeply declined in the drastic depopulation in the Greek territories. Of course, this was also the time of the explosive growth of Christianity in precisely those territories, a development that Plutarch never mentions, but which certainly played a role in redirecting worship and funding. Further affecting the situation would be the elimination of the city-state democracies, and thereby the Amphictyonic League which supported Delphi, particularly with the advent of the Roman Principate. That is, with the possibility of patronage being limited by the Emperor and a small elite controlling the economy, Delphi and other formerly prestigious sites will have vied for the attention of the few funds available, with the bigger names winning out. Everyone knew Delphi and Dodona, but who in Rome knew Tegyrae or Ptoüm? Most likely, a combination of all of the above-mentioned points was responsible for the silencing of the oracles.
Even Delphi went silent in the late fourth century with the closing of pagan temples by Emperor Theodosius. This was, in a sense, a very real triumph of the Nazarene over the Delian.