"Teacher, which is the great commandment in the Law?"
And He said to him, "'You shall love the LORD your God with all your heart, and with all your soul, and with all your mind.' This is the great and foremost commandment. The second is like it, 'You shall love your neighbor as yourself.' On these two commandments depend the whole Law and the Prophets."
From a Biblical perspective, the most important thing a follower of Jesus is supposed to do is love God and love people. Anyone who does that will think, talk, and act in ways that demonstrate that love.
As Jesus said in Matthew 12:34-37:
"For the mouth speaks out of that which fills the heart. The good man out of his good treasure brings forth what is good; and the evil man out of his evil treasure brings forth what is evil. And I say to you, that every careless word that men shall speak, they shall render account for it in the day of judgment. For by your words you shall be justified, and by your words you shall be condemned."
What we say is symptomatic of the state of our hearts. Someone who unthinkingly shouts "Jesus Christ" when he hits his finger with a hammer is demonstrating something about his view of and respect for Jesus. If I constantly said "Lily" the way many people say "God," I'd quickly find myself with some marital tension!
The Bible also talks about "mockers" or "scoffers," people who try to make a joke of God and of the existence of right and wrong. The Bible doesn't tell us to avoid very many kinds of people, but mockers or scoffers are folks we're encouraged to steer clear of, in places like Psalm 1:1 and Proverbs 22:10 (to give just a couple of examples). Note that these aren't just folks who enjoy joking around, but people whose speech and attitudes don't demonstrate respect for God and His Word.
The Proverbs talk a lot about different kinds of speech, both wise and foolish, good and bad. So does the New Testament. A big part of the emphasis is on things that are hurtful or dishonest like lying, slander, gossip, flattery, manipulation, insults, and just plain meanness. Leviticus 19:14 is a fascinating but little-known example of the latter: "You shall not curse a deaf man, nor place a stumbling block before the blind, but you shall revere your God; I am the LORD." That's the kind of thing Colossians 3:8 is talking about: "But now you also, put them all aside: anger, wrath, malice, slander, and abusive speech from your mouth."
Jesus also makes it clear that our attitude toward one another is a major issue that shows up in our speech. In Matthew 5:21-24 He says,
"You have heard that the ancients were told, 'You shall not commit murder' and 'Whoever commits murder shall be liable to the court.' But I say to you that everyone who is angry with his brother shall be guilty before the court; and whoever shall say to his brother, 'Raca,' shall be guilty before the supreme court; and whoever shall say, 'You fool,' shall be guilty enough to go into the fiery hell. If therefore you are presenting your offering at the altar, and there remember that your brother has something against you, leave your offering there before the altar, and go your way; first be reconciled to your brother, and then come and present your offering."
"Raca” meant "empty head," or in today's language something like "idiot" or "moron." So it's like He's saying, "Don't be too proud of yourselves just because you have never killed anyone. If you get angry and start calling somebody names, your heart is no more full of love than a murderer's. Don't think you're on good terms with God if you refuse to try to patch up your relationships with other people." James 3:8-12 addresses the same thing with a series of illustrations.
But what about what we would call profanity? Here are some passages worth considering:
Let no unwholesome word proceed from your mouth, but only such a word as is good for edification according to the need of the moment, that it may give grace to those who hear. And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice. And be kind to one another, tender-hearted, forgiving each other, just as God in Christ also has forgiven you. (Ephesians 4:29-32)
Let your speech always be with grace, seasoned, as it were, with salt, so that you may know how you should respond to each person. (Colossians 4:6)
Let no one look down on your youthfulness, but rather in speech, conduct, love, faith and purity, show yourself an example of those who believe. (1 Timothy 4:12)
Likewise urge the young men to be sensible; in all things show yourself to be an example of good deeds, with purity in doctrine, dignified, sound in speech which is beyond reproach, in order that the opponent may be put to shame, having nothing bad to say about us. (Titus 2:6-8)
What we call profanity is a subtle issue. It's an indicator of what's in our hearts, and it communicates something to those around us.
Once after we had spent most of the day fishing with the husband of a distant relative, he asked Lily, “You’re Cuban, right? I mean, I know you were born here, but you were raised in a Cuban home, right? So how come you don’t cuss?”
He recognized that she didn’t talk the way many people of his acquaintance did. That opened the door for a great conversation about spiritual things!
The issue of profanity has less to do with avoiding particular words than it is about doing what Paul discusses in Galatians 5: walking by, being guided by, and yielding the fruit of, the Holy Spirit. When God changes our hearts through faith, and as we invite Him to continue changing us, our behavior and speech naturally change. We become more sensitive to both God and others.
As Paul points out to Titus, the goal is that our speech would be sound and irreproachable. It would take some serious mental gymnastics to argue that profanity fits that description.