Matthew 12:33-37
33 Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
34 O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.
35 A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
37 For by thy words thou shalt be justified, and by thy words thou shalt be condemned.
The Grace New Testament Commentary - Matthew 12:33-37
12:33-35 The words the Pharisees speak reveal what is in their heart. Just as fruit indicates the nature of a tree, so their words reveal the nature of their hearts (cf. Matthew 7:15-20). Jesus compares them to vipers. Just as the dangerous and deceptive actions of vipers expose their nature, so the venomous and insulting words against the Holy Spirit disclose what the Pharisees had become. What the heart is filled with (i.e., its abundance) and values most (i.e., its true treasure) cannot be hidden. The mouth will speak words, whether good or evil, with reference to the testimony the Holy Spirit has provided.
12:36-37 The "day of judgment" may refer both to the believer's and the unbeliever's time of judgment, the Judgment Seat of Christ and the Great White Throne Judgment. If so, "by your words you will be justified" refers to vindication and "by your words you will be condemned" refers to nonvindication (katadikaioĊ). Both believers (2 Cor 5:10) and unbelievers (Rev 20:11-14) will be judged to determine their degree of reward and suffering respectively.
"Every idle word" of the Pharisees would be used as evidence against them "in the day of judgment." Their unbelief generated careless words without substance. They had no basis for concluding Jesus was utilizing demonic power. Whether one is justified or condemned ultimately depends on whether he or she believes in Christ (John 3:18; Rom 3:28). However, words exhibiting perpetual unbelief will later vindicate God's judicial decision to all creation.
Thomas Constable's Expository Notes on the Bible - Matthew 12:37
12:37 This verse sounds as though it may have been proverbial, or perhaps Jesus made it a proverb here. The context clarifies that the justification and condemnation in view deal with God passing judgment on everyone. Obviously, Jesus did not mean that if a person was able to say all the right words, he or she could deceive God and win salvation by clever speech. The basis of justification and condemnation is faith, but words reveal character, so they become the instruments by which God judges.