Salvation vs Condemnation: Matthew 12

But those who reject Christ, who curse him, for instance, would not be accepting his gift.

So there are two points:

1. If it is as he says, out of the mouth the heart speaks, those who curse God with their words may be revealing that they do not accept him in their heart, which is to say, they do not accept the salvation that he offers them. This is directed at all people - particularly the Pharisees - who were supposedly holy people, but would curse Christ and condemn him to death. How could any of them claim to be holy if they were condemning God with their mouth? And furthermore, will God accept those who do not accept him?

2. Even more subtly, the words you speak affect how you act; so one who speaks ill and curses all of his life will fill himself with evils and set his heart on wickedness. A person should not expect for themselves that they can live in such a way and then have a sudden turn around just in time to accept the thing they have been rejecting all of their life to fulfill their desires?

This appears to be addressed to all readers and hearers, but it is not to be applied by the reader or hearer to another person besides themselves.

Quick answer

This message is for the Pharisees, whom Jesus is rebuking for their self-righteousness. The Pharisees were very confused about their spiritual condition, so Jesus was trying yet again to show them their wickedness and need for a Savior. There is no conflict between this passage and salvation by grace.

Explanation

The Context of Scripture

This might go without saying, but the spiritual truth that God is trying to communicate through scripture is a lot "bigger" and is very "different" than the way we generally think of things. (Isaiah 55:8-9) Throughout the Bible, I get the sense that God is continually trying to "explain it a different way..." or "in other words..." or "approach it from a different angle", in hopes that once we have seen truth from all of these different angles, we might finally have a clear picture of what reality actually looks like from God's perspective. (e.g. Hebrews 1-2)

For this reason, I think it is important to seek "God's meaning" by careful study of all of the relevant passages, rather than focusing too much on the details of word definitions, etc. (Matthew 4:1-8)

"Bad Tree!"

Jesus is clearly trying to make a point by drawing a contrast between "good trees" and "bad trees". He is talking to a very confused group of people (the Pharisees) who think that being "religious" is the same as being "good". Jesus, once again, is trying to explain that they are not "good". This time he is pointing out the wicked things that are coming out of their mouths, and indicating that they will be judged for every last one of them. (Consider the context of the passage - blaspheming the Spirit.)

Salvation

Is Jesus teaching "salvation by words"? Not really. (Mark 7:6-7, James 2:14-17) If He was, we would all go to Hell. (James 3:2, 2:10) We should understand this passage in the same way that we understand the law... it is the perfect standard of God, which we should all live up to, but which no man actually does live up to, and which we will all be judged by... (James 2:10-11, Matthew 5:48) ...unless we return to Him and are cleansed from our sin by His gracious gift of salvation via the sinless sacrifice of Christ. (Romans 3:19, Galatians 5:18)

Judgment Day

I may be wrong in this, but to me it looks like "judgment day" is reserved for the wicked, and the "righteous ones" aren't there for that event... (Revelation 20:4-6, 11-15, 2 Peter 3:7, John 3:18)