When it comes to the issues of swearing and cursing, the Bible makes it extremely clear, "Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs that it may benefit those who listen." (Ephes. 4:29)
James also addresses the issue by stating, "With the tongue we praise our Lord and Father, and with it we curse men, who have been made in God's likeness. Out of the same mouth come praise and cursing. My brothers, this should not be. Can both fresh water and salt water flow from the same spring? My brothers, can a fig tree bear olives, or a grapevine bear figs? Neither can a salt spring produce fresh water." (James 3:9-12)
What James is saying is that the lives of Christians should not be characterized by evil speech. By making the analogy of both salt water and fresh water coming from the same spring (which is uncharacteristic of springs), he makes the point that it is uncharacteristic for a believer to have both praise and cursing come from his/her mouth. Also it is not a characteristic for us to praise God on one hand and curse our brothers on the other. This, too, is uncharacteristic of a true believer.
Jesus explained that what comes out of our mouths is what fills our hearts. Sooner or later, the evil in the heart comes out through the mouth in curses and swearing. However when our hearts are filled with the goodness of God, praise for Him, than love for others will pour out from our mouths. The truth is, our speech is a strong indicatication of what is in our hearts. “The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks” (Luke 6:45).
So what's the deal with swearing? When we swear and curse, we are giving evidence of the polluting sin in our hearts that must be confessed and repented of. Thankfully, our great God is “faithful and just and will forgive us our sins and purify us from all unrighteousness” (1 John 1:9). When this happens, we receive a new nature from God (2 Corinthians 5:17), our hearts are transformed, and our speech reflects the new nature God has created within us.
