Speaking evil things is always a sin.  More of God’s children might be lost for neglecting to control the tongue than anything else.  It is easy to speak without thinking, and that often results in sinful speech.  The Bible is clear about the importance of not misusing our tongues.

Matthew 12:36-37 (ESV)
³⁶ I tell you, on the day of judgment people will give account for every careless word they speak, ³⁷ for by your words you will be justified, and by your words you will be condemned.”

Ephesians 4:29 (ESV)
²⁹ Let no corrupting talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear.

Colossians 3:8 (ESV)
⁸ But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth.

James 3:6 (ESV)
⁶ And the tongue is a fire, a world of unrighteousness. The tongue is set among our members, staining the whole body, setting on fire the entire course of life, and set on fire by hell.

These are only a select few of the many passages that condemn sinfully using the tongue.  If misusing the tongue is frowned upon by God, how much worse must it be when the tongue is used sinfully against our own brethren?  James warns us about this when he said, “Do not speak evil against one another…”

How do you speak to and about your brothers and sisters in Christ?  There is no doubt that we get angry with each other from time to time, but that is no excuse to speak evil about one another.  If we do, it shows that we lack the proper love for one another, and that will keep us out of heaven.  Some of the saints at Corinth lacked love for their brethren and demonstrated it by their evil speaking.

2 Corinthians 12:20 (ESV)
²⁰ For I fear that perhaps when I come I may find you not as I wish, and that you may find me not as you wish—that perhaps there may be quarreling, jealousy, anger, hostility, slander, gossip, conceit, and disorder.

There is a long list of passages that condemn slander, especially against those who are a part of our spiritual family in Christ.  Peter also warns us about it.

1 Peter 2:1 (ESV)
¹ So put away all malice and all deceit and hypocrisy and envy and all slander. 

Let us not deceive ourselves into thinking that we can speak evil against our brethren and make it all alright by praising God on Sunday.  James reveals the futility of such when he wrote about the tongue.

James 3:9-12 (ESV)
⁹ With it we bless our Lord and Father, and with it we curse people who are made in the likeness of God.  ¹⁰ From the same mouth come blessing and cursing. My brothers, these things ought not to be so. ¹¹ Does a spring pour forth from the same opening both fresh and salt water?  ¹² Can a fig tree, my brothers, bear olives, or a grapevine produce figs? Neither can a salt pond yield fresh water.

If your brother or sister is doing something you do not like, or you think needs correcting, do the right thing and go to them with it.  Slandering, backbiting, gossiping, speaking evil against, etc., will not help them change.  It will only cause you to fall out of favor with the God you both serve.

As you wind down for the night, think about these things.