Idle words are those that are useless, malicious, have wicked intent, or words that accomplish nothing. Matthew Henry highlights the fact that the heart is the fountain and words are the streams flowing from the fountain. "A troubled fountain, and a corrupt spring, must send forth muddy and unpleasant streams."
Jesus told the Pharisees that men will have to answer for every idle word that passes their lips in Matthew 12:36. In context these religious leaders had just accused the Son of God of doing the devil's work. Jesus warned them that everything will be forgiven mankind except the blaspheming of the Holy Spirit. I have often heard young people express fear of mentioning the Holy Spirit for fear of committing this unforgivable sin. Personally, I do not believe that a person who is a true believer could ever commit this sin because our hearts are indwelled by the Holy Spirit; and according to Paul's second letter to Timothy (2:13) He cannot deny Himself.
A true believer deeply desires to please God, and we have been given a new heart with the potential to be filled with God's desires. The journey is all about being sanctified and becoming more like our Father. However, Jesus' admonition to the Pharisees is perplexing. If we are forgiven all our sins when we accept Jesus as our Savior, then how can we stand before Him on the Day of Judgment and account for all our idle words? It seems to be a conundrum.
There are two ways of understanding Numbers 22:20-22. Firstly, God says, “If these men call you in the morning, then go with them and do as I say do and only that.” Secondly, God says, “I give you up to your own lusts, Balaam. Since they came go ahead and go with them.” In both cases, God's anger is kindled and burns brightly against Balaam for several reasons.
In the first reading, Balaam disobeys God because he does not wait for the princes to call him which is direct disobedience to God's command. In the second, God's directive is similar to what Paul talks about in Romans chapter one. God gives up the corrupted, wicked to their own lusts and it brings down His wrath upon them for He has provided everything for them to believe in Him, yet they turn from Him to pursue fleeting pleasures trading their souls for temporary satisfaction. In either case, God knew what would happen and how Balaam would bitterly betray Israel to Balak.
Regardless of how you interpret these verses, it was a sin of Balaam to go with the princes of Moab because God had originally commanded him not to go to Moab. As Matthew Henry says, “As God sometimes denies the prayers of his people in love, so sometimes he grants the desires of the wicked in wrath.” Nevertheless, Balaam had spoken to these princes, “If Balak would give to me his house full of gold and silver, I could not go beyond the mouth of Jehovah my God, to do anything, small or great” (v. 18). Does Balaam think he can change the mind of God? He calls Him “my God” as if he had a personal relationship with Him. How deeply wicked it is to know God's will (not to go to Moab, and not to curse Israel), yet to turn around for the sake of a few coins and ask permission to go when he was commanded not to go.
Balaam said he would not go against what God said do, yet he did just that. This is what Jesus was talking about when He warned what would happen to those who spoke idle words. How often do we say things that we don't mean, and give promises we don't fulfill? These words Balaam spoke came from a vain and worthless heart. Jesus said the bad tree bears bad fruit, and the good tree bears good fruit. Also, what comes out of the mouth is what is buried in the heart, and therefore leads to good deeds or wicked deeds. Balaam's word should indicate a penchant to obedience but his actions indicate the opposite.
If Balaam had meant what he said, the more honorable princes sent by Balak would have been sent back before they had ever dismounted or spoken to him about all manner of honor to be given him. Idle words mean nothing, do not profit anyone or edify, and are an abomination in God's sight. If we do not have our Advocate Jesus by our side when called to account for our idle words (for we have all spoken them), then we are ruined.