I’m seeing claims here and there that the Christian churches (i.e. Catholic and Orthodox) taught that people who habitually wrote with their left hand were in some sense or other “of the devil,” or an “abomination,” and that they were even explicitly punished for using their left hand, i.e. imprisoned or tortured or whatever.
That was common practice in schools of all kinds. I don’t think it has anything to do with religion; more to do with ideas about education and child development.
This would only signify prejudice antedating Christianity if we knew that pre-Christians used sinister to mean, well, “sinister” or something similar, as opposed to just using it o to mean “left”. Do we know this?
The negative connotations of the word “sinister” in English do not predate the fifteenth century - but, then, neither does the word itself.
English acquired the word from French, and it’s possible that it came into the English language ready-fitted with negative connotations. But, even so, we’d need to establish when it acquired those connotations; we can’t just blithely assume that they pre-date Christianity because the root word does.
Wikipedia says that sinister had acquired its negative senses by the “Classical Latin” period - i.e. about the time of Christ, but in a culture which was definitely not Christian. Furthermore, it says the Romans acquired these attitudes from the Greeks.
If there is evidence of a bias in the Judeo-Christian tradition, it’s probably more of a positive bias towards the right rather than a negative bias towards the left (though, you might say, those are two sides of the same coin). So, Jesus is said to be seated at the right hand of the Father, for instance.
I can’t off-hand think of any evidence of theological bias in relation to left-handedness, which is what the OP asks about, as opposed to left as a relative location. But, obviously, if you mentally place bad things on the left and good things on the right, even figuratively, it’s not a huge leap to think of left-handed people as having a bent towards evil or weakness.
I have a scar on my left hand from school because of a teacher striking me with objects any time I did anything lefthanded.(or even leftfooted, if I was standing and put my left foot forward first when I started to walk I got hit and made to start over like “a proper child of christ”) she eventually got canned for that shit, this was a public school.