Historically, the Church acquired some of its doctrine and most of its ritual from non-Christian sources such as Mithraism in the early centuries.
The explanation for this has traditionally been that the practices were sanctified by being brought into the Church, rather than that the Chuuch was corrupted by the introduction of pagan rituals.
If it is entirely right and proper to "sanctify" Greek and Persian pagan practices of 200 to 400 AD by using them in Church ritual, why not Hindu practices of 2005?
Carried too far, of course, this would result in Catholicism becoming only another of the thousands of sects of Hinduism.
...and thus our conclusion: In not one instance has Acharya made a convincing case that Christianity borrowed anything from Mithraism. The evidence is either too late, not in line with the conclusions of modern Mithraic scholars, or just plain not there. Acharya will need a lot firmer documentation before any of her claims can be taken seriously.--- (Mighty Mithraic Madness - Did The Mithraic Mysteries Influence Christianity?)
Our knowledge is mostly ingenious guess-work; of the real inner working of Mithraism and the sense in which it was understood by those who professed it at the advent of Christianity, we know nothing. Some apparent similarities exist; but in a number of details it is quite probable that Mithraism was the borrower from Christianity. ... It is not unnatural to suppose that a religion which filled the whole world, should have been copied at least in some details by another religion which was quite popular during the third century. Moreover the resemblances pointed out are superficial and external. --- (Mithraism entry in Catholic Encyclopedia)
In summary, the argument that Mithraism was a source for Christian beliefs or practices falls on three ground. First, there would have been no reason for Christianity to have taken anything from Mithraism. Second, the elements of Mithraism generally put forward as similar to Christianity are either untrue or in-significant. Third, and most deadly, Mithraism actually appears after Christianity.Did Christianity steal from Mithraism? Most definitely not. --- (What Mithraism Isn't)
Ultimately, all attempts to prove Catholicism "pagan" fail. Catholic doctrines are neither borrowed from the mystery religions nor introduced from pagans after the conversion of Constantine. To make a charge of paganism stick, one must be able to show more than a similarity between something in the Church and something in the non-Christian world. One must be able to demonstrate a legitimate connection between the two, showing clearly that one is a result of the other, and that there is something wrong with the non-Christian item.In the final analysis, nobody has been able to prove these things regarding a doctrine of the Catholic faith, or even its officially authorized practices. The charge of paganism just doesn’t work. --- (Is Catholicism Pagan?)