Fair enough, and I appreciate your spirit of ecumenism, but most of the traditions we associate with Hallowe’en (trick-or-treating at houses, pranks against home-owners, costuming and going door-to-door) actually came from later English folk celebrations associated with Christmas, which gradually became assimilated into the Hallowe’en traditions. Some of the traditions that these were associated with the older folk traditions associated with Samhain were after-the-fact claims by folklore popularizers that lack any primary textual basis in scholarship.
Our primary texts referring to Samhain date, at the earliest, from well after the beginning of the Christian era, beginning in the 10th century, and were written down by Catholic monks from oral traditions - so from an anthropological standpoint, there is a possibility that they incorporated Catholic beliefs (such as an association with a feast celebrating those who died, as with All Soul’s Day into an event that had no such associations.) No primary textual evidence connects Samhain with a festival of the dead.
The idea that it began as a festival of the dead and was later “appropriated” as a festival of the dead by Christians from a pagan tradition comes from Sir Edmund Frazer, who theorized that about most holidays and claimed, essentially, that many cultures have festivals of the dead, that Christians had appropriated other pagan holidays (a belief in which he was largely incorrect), and so Samhain must have been a festival of the dead that the Church appropriated. The Church, however, had been celebrating days commemorating all the martyrs since the 4th century on different dates (on Easter Week in Syria, the Sunday after Pentecost by the Greeks, May 13 in Rome.) All Saints Day was being celebrated in the British Isles A.D., but the Irish bishops celebrated it on April 20. In 998 Bishop Odilo of Cluny began the tradition of a mass for the souls of all the dead in the month February, which was later moved to November 2, its current date. So, the first primary textual evidence we have for a religious service associated with the dead at this time is Catholic, not pagan. (There were pagan festivals of the dead associated with other seasons and non-Celtic pagan traditions, such as Lemuria.)
Samhain also probably had little to do with the harvest and was probably not a religious festival at all, as even Frazer noted that it was more likely a time when Celtic people naturally gathered due to the need to drive cattle from the highland to the lowlands, and it was a good time for the tribes to gather to conduct business. A lot of Frazer’s ideas have been disproven based on later scholarship but I think he was on the right track here. As Skadi probably knows, “Samhain” wasn’t a god, it was a time of year - the word literally means the period we call November. It wasn’t really a harvest festival for the Celts, as many claim, as many of the Celts of the time weren’t notably farming people and the period at the end of October to the beginning of November have no particular significance to agricultural people.
Some Hallowe’en folk traditions probably are part of Celtic folk traditions, such as divination (which good Catholics aren’t supposed to do), and lighting bonfires (associated with Beltane), but others, such as feasting on slaughtered cattle (both common in any large gathering in pre-electric times), aren’t really a part of Hallowe’en celebrations. The pagan custom of sacrificing infants by smashing their heads against a rock to celebrate the pagan deity Crom Cruach (a deity later borrowed by Robert E. Howard for his Conan series), as in the Dindsenchas texts, has been associated with the celebration of Samhain, but the scholar Ronald Hutton has argued in Stations of the Sun: A History of the Ritual Year in Britain. Oxford, Oxford University Press ISBN 0-19-288045-4 (1996), that there were no particular religious associations with Samhain other than as a tribal gathering time (more administrative than religious, although there were undoubtedly religious activities at the time), and that the stories from the 10th century simply used Samhain as a setting for stories about the activities of pagan gods and great warriors in the same way later stories about King Arthur’s court were set at Christmas or Pentecost - it was a logical time to set a story, as those were times when the herding Celts would be together.