I’m surprised no one’s quoted James yet. I used this passage once when a friend asked if the Bible condemned swearing:
But no man can tame the tongue. It is an unruly evil, full of deadly poison. With it we bless our God and Father, and with it we curse men, who have been made in the similitude of God. Out of the same mouth proceed blessing and cursing. My brethren, these things ought not to be so. Does a spring send forth fresh water and bitter from the same opening? [James 3:8-11]
Now granted, I don’t think any one’s going to lose salvation because one time back in 1977 they got a little upset and said the word “damn,” However, if you’re swearing a lot and don’t seem able to control it, you need to reevaluate yourself - what “slips out” reveals what hides underneath. I know when I returned to Christ I had to get a grip on my loose tongue big time.
But those things which proceed out of the mouth come from the heart, and they defile a man. [Matthew 15:18]