Yes, things are indeed incorporated into Christendom. Yes, once they become habitually practiced for a number of years, they become traditional. If something pre-exists the practice of Christianity in a given culture, and is added to the practice of Christianity in that culture at a later date, then it has, in fact, been added.
I'm not implying anything negative......I'm an atheist and I still do Christmas, because it is "traditional" for our family to get together and exchange gifts around a decorated tree and have a big meal, etc. It is a formal holiday, school is out, people have time off from work, all that makes it easier to gather.
But you will not find any of the elements of Christmas in the first century Christian community. It was a small Jewish cult that has grown through proselytizing and force of military might. Nobody knows when Jesus was born, and most scholars will tell you that it was not in December, but in warmer months. The birth date of Christ was set by an emperor to align with the Saturnalia which helped incorporate another religion into the celebration. People already celebrated Saturnalia.
Would you care to share a recipe for Christmas pudding? Much of the world, like here in the U.S., knows nothing of Christmas puddings. I would find it very interesting to try next year.
There are lots of recipes but this one I have used many times and I know is quite widely liked:-
It's quite a long list of ingredients but fairly easy to make once assembled. Quantities for 2 puddings, given in Imperial units (the recipe is old enough to still use oz). The ingredients are given in the order in which it is recommended to add them, mixing at each step or couple of steps as common sense dictates.
8oz suet
1 heaped teaspoon mixed spice
1/2 teaspoon grated nutmeg
1/4 teaspoon ground cinnamon
4oz self-raising flour (flour plus baking powder - standard in the UK but don't know elsewhere)
1lb soft brown sugar (I use 3/4 to make it less sweet)
8oz breadcrumbs grated from a stale loaf (doesn't have to be completely stale)
8oz sultanas (I find these fairly pointless so substitute chopped prunes)
8oz raisins
1 1/4 lb currents (1lb is fine)
2oz chopped almonds (I use 4oz because I like them)
2oz mixed candied peel (I use 3oz)
grated rind (zest) of 1 orange and 1 lemon
1 apple peeled, cored and diced (I use a cooking apple for more acid to balance overall sweetness)
Once these dry ingredients are mixed, you mix up separately :
4 beaten eggs
10 floz stout (Guinness or equiv)
4 tablespoons rum (I substitute 8 tbsp cognac or armagnac, as it goes better with the prunes)
and pour this lot over it.
Then you stir it all together thoroughly by hand (don't use a machine or you may break up the fruit into mush)- this takes a few minutes: important to ensure all the dried fruit and sugar is wetted so it can soak up the liquid, and leave overnight covered with a cloth for this to occur.
You then butter 2 pudding basins, fill them right to the top with the mixture, and tie over a sheet of greaseproof paper and some Al foil (the latter helps the string to stay in position just below the lip of the basin - you want a decent seal). Then you put them in a large covered pan with gently boiling water coming about halfway up the basins, and steam/boil them for 8hrs, topping up the water when necessary. Then the puddings, still in their basins, go in the cellar or somewhere cool for at least 2 weeks, preferably 4, before use. (I once made them only a week before and the taste was not as good.) Don't be alarmed if you get an oily layer of suet on the water at the end. This is normal.
They need to be reheated and served hot. Either 2hrs in another boiling water pan (best), or possibly microwaved - but in the latter case I would turn them out of the basin first - the outer part can overheat, dry and stick to the basin so it doesn't come out in one piece as a nice dome. (This happened when I took one to my brother year before last.)
Classically you serve the pudding flambeed, by turning it out onto a warmed serving plate, spooning over cognac with a pre-heated spoon, and setting fire to it before carrying it, with a certain degree of ceremony, to the table.
Brandy butter is the traditional accompaniment but I prefer to make brandy cream - just whipped cream with a teaspoon of sugar and some cognac in it.
It is heavy, from the days long before central heating, so serve in modest portions. But pretty good, provided your guests have not OD-ed on turkey etc. I took some to France last year and the French family ate almost the whole thing.