Salvation vs Condemnation: Matthew 12 Matthew 12 ESV


  A Tree Is Known by Its Fruit
  
  33  “Either make the tree good and its fruit good, or make the tree
  bad and its fruit bad, for the tree is known by its fruit. 34  You
  brood of vipers! How can you speak good, when you are evil? For out of
  the abundance of the heart the mouth speaks. 35  The good person out
  of his good treasure brings forth good, and the evil person out of his
  evil treasure brings forth evil. 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.”


In this context, does being condemned by the words you speak conflict with salvation by grace?  So if you're saved by grace, Jesus would step in and say, "he accepted my sacrifice, his sins are forgiven."  Who would this message be for?  Non-believers would go to hell if their words are good or bad.
I think Jesus is trying to say a few things here.


Given the context, I think he's warning against hypocrisy. He calls his listeners a "brood of vipers" - they are hypocrites for trying to sound good when, at the heart, they are evil. They are painting their bad fruit to look good. 
Watch your mouth. Like James, ("the tongue is a fire"), he warns that even a careless word of spite or a biting remark is a sin, a sin against a holy God, and as such it warrants punishment. On an eternal scale, sinful words are no different from sinful deeds.
Heart ⇒ Mouth. What you say comes from what you think, which comes from who you are. Ultimately, evil or unkind words only come from an evil person. 


So Jesus isn't contradicting salvation by grace; rather, he's proving its premise: depravity. Everyone says things they shouldn't, and if evil words really come from an evil hard, everyone is sinful at heart and deserves God's wrath and condemnation. 

But grace offers a change of heart. Because Jesus had a perfect heart, he never spoke a wrong word. He always spoke good, and his record of righteousness is transferred to us. Not only do we get a clean record (so God looks at our words and sees Christ's), we also get a new heart so that we begin to speak without evil.