Well, I have my days...
"Ah, but good Christians would never swear, would they"
Paul tells us to "refrain from strong language..." He didn't say it was absolutely forbidden to use it even when appropriate.
When Christ accused the priests of being "whited sepuchres full of dead men's bones and full of corruption", that was considered one of the worst swear terms a man could utter to another, the equivalent of being told one was Full Of S----!