In The Natural History of Religion, Hume argued that monotheism is not humanity's natural religious impulse. Monotheism is, rather, an intellectual achievement resulting from a coherent and rational view of the universe. It is also always in danger of the corrupting influence of "the vulgar" - that mass of humanity that is incapable (for a variety of reasons) of such intellectual achievement. The history of religious thought is the story of the oscillation between the intellectual and the vulgar. Edward Gibbon picked up this idea from Hume and from there it became part of the intellectual furniture of modern people (including me, who have never read Hume or Gibbon at any length).
This "two-tiered" model has as one of its premises that popular religion is an unchanging collection of prejudices and superstitions permanently lodged in the brains of the unsophisticated. The religion of the intellectuals may change, but the religion of the vulgar does not. Another premise of the model is that changes in religious beliefs or practices can often be explained by changes in the power dynamics between the elites and the masses, e.g., polytheism increases as the masses grow more powerful (or the elites grow weaker).
It's easy to see how this model has been applied to the rise of the cult of the saints. One theory is that the elites lost much of their power in the crisis of the third century, which opened the way for superstitious fears and practices. Another theory is that mass conversions when Christianity was made the state religion forced the leadership to accept the pagan practices that the new converts brought with them. Whatever theory is correct, the idea is that the power of popular religion increased and the cult of the saints (as a form of polytheism, ancestor worship, etc) rose with it.
But what is known about the history of the cult of the saints does not conform to this model. First, as to the question of elites versus the masses, the Christian leaders knew that when they were formulating dogmas that many of the laypeople would find them difficult to understand. On the other hand, both the elites and the regular folk shared religious practices. There was no significant difference based on class or education. Bishops celebrated masses in the presence of holy graves and relics (something that was deeply disturbing to non-Christians - but more on that momentarily). In fact, the importance of holy graves was something that distinguished Latin Christianity from both Judaism and Islam. There were holy graves in Judaism and Islam, of course, and they were important - but they were to the side, so to speak, of mainstream belief and practice. They were never fully embraced by the religious leadership. That tension did not exist in Latin Christianity.
More importantly, though, the cult of the saints - far from being a resurgence of popular religion - represented a distinct break from traditional Mediterranean beliefs and practices concerning the dead. Brown describes those beliefs in this way:
One thing can be said with certainty about the religion of the late-antique Mediterranean: while it may not have been markedly more "otherworldy," it was most emphatically "upperworldy." Its starting point was belief in a fault that ran across the face of the universe. Above the moon, the divine quality of the universe was shown in the untarnished stability of the stars. The earth lay beneath the moon, in sentina mundi - so many dregs at the bottom of a clear glass. Death could mean the crossing of that fault. At death, the soul would separate from a body compounded of earthly dregs, and would gain, or regain, a place intimately congruent with its true nature above the earth in the heavy clusters of the Milky Way. Whether this was forever, or, as Jews and Christians hoped, only for the long hiatus before the resurrection of the dead, the dead body joined in the instability and opacity of the world beneath the moon, while the soul enjoyed the unmovable clarity of the remainder of the universe.As Brown states, Christians also held this belief about the "fault line". The graves of the martyrs and saints, however, were places of contact between heaven and earth. The departed saint was believed to be specially present at the grave, as proven by the presence of a quality of holiness and power characteristic of the saint. The belief of the masses was that heaven and earth did not join in this way. The Greek and Roman cult of the heroes (to which the cult of the saints is often compared) isn't even that similar. The worship of the gods was kept distinct from the cult of the heroes, unlike in Christianity. And for Christians, it was precisely because the departed were human beings with a close relationship with God that they were effective intercessors for the living. That is an idea utterly foreign to the cult of the heroes.
The cult of the saints, which involved physical veneration of relics and bodies, also violated common taboos against the touching of dead things. Cemeteries were often located outside cities because the people wanted to keep the dead at a distance. Christians, on the other hand, started building shrines in the middle of these cemeteries, sometimes to the extent that they became cities outside of the cities. "Tomb and altar were joined." Shrines and graves of the saints became public places, as opposed to the common belief that graves were private places for the family of the deceased.
The cult of the saints, therefore, was neither a point of tension between Christian elites and commoners nor was it a resurgence of the "religion of the vulgar". Now, whether we accept the cult of the saints is another question - but at least we can be reasonably sure it wasn't mere polytheism.
Hopefully I'll have more to say on this subject as I make my way through the book.