You are right, it gets worse. All of the various Mystery Religions of the Roman empire were absorbed into the Catholic apparatus making an even bigger mess.

In The Gnostics and Their Remains by C.W. King, we see Emperor Hadrian saying the quiet part out loud that Christ and Serapis were one and the same. Christianity arose from Mithraism, and the part involving Mithraic priests and Augustine's shows that there was intermingling.

The Emperor Hadrian, a most diligent inquirer into things above man's nature, got himself initiated into one mystery after another; nevertheless we shall find him writing from Alexandria that the worship of Christ and of Serapis was in that city one and the same, and moreover the sole religion of that immense population. Consequently, those initiated into the true secrets of the old religion must have recognised the fact that their deity, whether the Sun or the Soul of the Universe, was nothing but a type of the One, the Saviour recently revealed to them: or else it would appear (which tells equally for our argument) that the new converts, in order to escape persecution, enjoyed their own faith under the covert of the national worship, which was susceptible of a spiritual interpretation quite cognate to their own ideas, [p. xviii] and indeed enshrouding the same. As for the worshippers of Mithras, their whole elaborate system of sacraments and degrees of initiation had no other object than the securing of spiritual enlightenment and spiritual blessings. The foundation being the pure teaching of Zoroaster, its holders were prepared gladly to accept any higher revelation, and to discover that the greater mystery had been foreshadowed in the types and ceremonies of the former one. In this way a man might continue a Mithraicist and yet accept all the doctrines of Christianity, as the priests of that religion in their last days assured the incredulous Augustine.