I don't see that. They say that "worshippers of Serapis and Christ were aware of each other and the differences within their religions" but would an non-worshipper see this?
This maybe entirely inaccurate, and I can't cite any reference for this, but many years ago, my grandfather told me that early Christians "stole" the dates of their significant occurrences from the existing celebrations and rituals of other religions.
Christmas is a great example.
Jesus' birth is supposed to take place around the 25th of December, but that would conflict with the relevant passages in the NT (Luke 2:8-20), which talk of shepherds in the fields watching their flocks at night. Well, the end of December is winter time. Shepherds did not have their sheep in the fields, they would be locked up in stalls or pens at this time of year. If HJ really existed, and Luke 2:8-20 was accurate, then HJ's birth would have needed to be sometime between, say, April and October. The selection of December 25 effectively hijacks the vastly older Pagan celebrations of the winter solstice, a time when many historical polytheistic religions celebrated the rebirth of the year as the days started to lengthen into spring. I think it is no accident that the date of the Christian "birth of the Son of God", effectively a new beginning, just happens to be placed in time right next to an existing, at the time well known and widely celebrated "rebirth". The giving of gifts, the decoration of trees, particularly Yule and Holly, and kissing under the mistletoe, all began with the traditions of Pagan religions, and in doing so ourselves, we are continuing
those traditions, which actually predate Christianity.
Easter is another example
Christians celebrate it on the Sunday immediately following the first full moon date after March 20. In selecting this date, they effectively hijacked celebratory dates of other religions;
Judaism. Jewish Passover is a holiday commemorating the Hebrews' exodus from slavery in Egypt. Due to differences in the way leap-years are dealt with in Judaism and Western Christianity, the calculation of the date is subtly different which leads to it not always coinciding exactly with Easter.
Pagan Religions. The Babylonian fertility goddess Ishtar (she actually had a number of different names in various cultures; Aphrodite, Astarte, Diana, Isis and Venus) had a son/consort called Tammuz, who was also known by a number of different names depending on cultures, Apollo, Hercules Horus, Krishna and Mithra. To the Babylonians, Tammuz was supposed to the God of the Harvest, and to the Sumerians, he was the shepherd-god, Dumuzid. The manner of his death changes depending on cultures, but they all talk of Ishtar creating a celebratory holy day (holiday) to honor
his death and then resurrection. (sound familiar?) Not surprisingly, this celebration, which went on for around 40 days, started around the time of the vernal equinox.
Ishtar / Easter: Coincidence? To this day, we still celebrate Easter with egg hunts (eggs being the symbol of birth) and rabbits (known for their fertility) and in doing so, are continuing, not a Christian tradition, but a Pagan one that began thousands of years before Christianity.