Matthew 12:36-37
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.
Does this verse teach that we will either get to Heaven, or avoid Hell, based on what we say?
What is the sense in which, "in the day of judgement", someone will be "justified" by their words?
We know that in the absolute sense, the Bible repeatedly says that we are "justified by faith" (Romans 3:28, 3:30, 5:1, Galatians 2:16, 3:24), and "justified freely by his grace" (Romans 3:24, Titus 3:7), because God is "the justifier of him which believeth in Jesus" (Romans 3:26):
Acts 13:39
39 And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses.
The Bible also teaches that "the day of judgment" for believers is different from "the day of judgement" for unbelievers, something covered in detail in this article. The "day of judgment" for a believer happens over one thousand years before the judgment on most unbelievers, because "the dead in Christ shall rise first" (1 Thessalonians 4:15-17), then Christ will rule for a thousand years with them, then the unsaved are resurrected for the Last Judgment (Revelation 20:4-6).
Therefore, it's likely that the "words" that Christ refers to that "justify" us is referring to the fact that a believer's words will be among the things that Christ uses to vindicate His decisions regarding the rewards that they receive at the Judgment Seat of Christ, where "every one may receive the things done in his body, according to that he hath done, whether it be good or bad" (2 Corinthians 5:10). This is where they will be vindicated before God, and fellow believers, based on the life that they lived, and has nothing to do with their forensic justification before God, which can only ever be based on faith (Ephesians 2:8-9).
See the aforementioned article for a more detailed discussion of that judgment on believers.
In what sense, then, are words the basis for condemnation?
For unbelievers, their sins of the tongue, including lying, will serve as a basis for their condemnation at the judgement, where "all liars shall have their part in the lake which burneth with fire and brimstone: which is the second death" (Revelation 21:8). And "who is a liar but he that denieth that Jesus is the Christ?" (1 John 2:22).
However, because "whosoever believeth that Jesus is the Christ is born of God" (1 John 5:1), believers that have the new birth will never come into eternal condemnation at any judgement.
Not only are they not condemned in the present:
John 3:18
18 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
But also in no future circumstance can believers ever be condemned in the eternal sense:
John 5:24
24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
Therefore, while unbelievers will be eternally condemned based on their words, believers, by virtue of their faith, are rescued from that condemnation.
The only "condemnation" that a believer could possibly receive would be a temporary condemnation at the aforementioned Judgment Seat of Christ, but that will result in loss of rewards, not going to Hell (1 Corinthians 3:15).
God, who knows everything, will use mankind's thoughts and words on the Day of Judgment to vindicate His decisions.
This is small comfort to unbelievers, who are "condemned already" (John 3:18). However, to believers rescued from condemnation, this is where Christ will examine your works done after you were saved to justify, before God and fellow believers, the extent to which you will be rewarded.