I was born a southpaw. My (adoptive) parents sent me to a small prep school at age 4, just before my 5th birthday. The school was run by two French women, mother and daughter, who were strict Catholics. Their reaction to my left hand dominance was, by modern standards, abusive. At mealtimes and during writing class my left mitt was either tied to my chair or I was made to sit on the offending hand. Their reasoning for this cruel and unusual treatment was based around their belief that left-handedness was a "work of the Devil" How absurd an idea is that?
It is possible to find the basis of this belief in the Latin words for left and right - Sinister (left) and Dexter (right) - but it is just superstition and ultimately nonsense. I also think it is related to Western writing going left to right, as opposed to Middle Eastern writing which scans right to left.
Scientific analysis says that 'retraining' in this way is way more damaging to the brain than just letting a southpaw bumble through a right-handed world.