Maybe I'm not good at metaphor, but I feel like this should be the other way round:
"How many times the Church has decanted the old wine of heathendom into the new bottles of Christianity."
Old cloaking itself as new.
The difference is intent. This isn't paganism hiding in Christianity. This is intentional organized decisions to convert pagan holidays to Christian ones. It is a radically successful attempt to eradicate them not "the old hiding in the new".
Christmas stands out even more as an example of it, literally the time of the celebration was chosen by committee to displace winter solstice holidays.
Plenty of religions have solstice holidays and and harvest celebrations, and likely other older religions did the same, but Christianity is the only one in living memory that actively subverts and attempts to consume the holidays of native populations they wish to convert.
To prevail against other similar beliefs, traits like intolerance for other gods, fear mongering with things like hell, etc, increase the chances of survival.
But I think your way of phrasing it matches history better. The Christians were trying to disguise the pagan sources for their holiday. Since the bottle or wineskin is in plain view and the wine inside is hidden, I personally would have compared the Christian veneer to the bottle and the pagan history to the wine inside it.