"But," in Paul's usage, isn't a preposition. And starting sentences with prepositions isn't considered "incorrect" by most grammarians[0]. Or even bad style.
If it's good enough for the Bible[1], it's probably good enough for you.
sighs No, but if you think I’m full of shit, I don’t really want to waste my time discussing how I developed this particular misunderstanding about the world for your amusement.
It’s been translated to the nth degree, is ancient & frequently obtuse on purpose...