Hi Bob. I have two somewhat related questions. In Jeremiah, the Lord tells the people not to say "the burden of the Lord". Could you please explain that, and also please explain Matthew 12:36-37 where we are told that we will "give an account" for the careless words we speak.
The Hebrew word masa`, translated in the KJV and some other versions as "burden", frequently refers to divinely inspired prophetic utterances. That is to say, it is a synonym for "the (specific) word of the Lord". That is the meaning in Jer.23:32-40. The false prophets are here claiming divine authority for their false prophecies by using this authoritative phrase, just as if they had said "The Lord told me '......'". Jeremiah is commanded to pass on to them an injunction against falsely claiming that the Lord "told them" when He hadn't spoken to them at all. It is shocking and somewhat amazing at how often in our own day one hears people attributing their actions to God in a similar way, but what is more appalling still is when people go so far as say that they are actually in conversations with God and are reporting His words (when in most if not likely all such cases this is not in fact true). I would say that this passage in Jeremiah definitely applies to such people, even though they may not have had a direct communication from God telling them not to claim that what they were making up was a direct communication from God. Either way, this is clearly an insanely dangerous thing to do, and was especially so for the false prophets of Jeremiah's day who received a special warning to desist. You might also check out these links:
The Prophets of Israel
As to Matthew 12:36-37, everyone will stand before the Lord in eternity to render an account for everything they have done. As believers, we shall have passed from death unto life, so that our evaluation is not a "last judgment" per se (Jn.5:22-24; and compare Rev.20:4-6 with Rev.20:11-15). We shall "stand before Christ's judgment tribunal" and render an account for our lives, all we have done and failed to do - that is what it is to fear the Lord (Rom.4:12; 2Cor.5:10). But our evaluation, while it may in some cases be extremely embarrassing, will result in salvation, even in those cases where nearly everything the person has done has to be burned as worthless (1Cor.3:11-15). We who believe in Jesus Christ will be like the servants of Luke 19:11-27 who have at least some production for our Lord - it may be 10 minas, it may be 5, or it may only be "interest" on our "talent" (corresponding to the three crowns of glory, life, and righteousness respectively), but we will have something to show for our Christian lives. And whatever we have done that is worthless will be burned up. So we need to notice first of all that the evaluation of the believer is fundamentally different from that of the unbeliever - there may be embarrassment, there may be loss, and therefore there may be reason to fear before the fact (cf. 2Cor.5:11 with the preceding verses), but all the promises of eternity will be ours forever, and once this judgment is completed, there will be "no more tears etc.".
Unbelievers, on the other hand, have no hope in this judgment, no matter what "good" deeds they have done, and these verses you ask about, Matt.12:36-37, indicate that even through their expressed opinions it will be an easy matter to demonstrate to every unbeliever the validity and justness of their condemnation. As James makes quite clear, the tongue is in many ways diabolical and its control problematic (Jas.3:6). For the tongue is the gate through which the thoughts of the evil heart first burst forth (cf. Matt.15:18-19; Lk.19:22; cf. Jer.17:9-10). So then we shall all have to give an account to the Lord (1Pet.4:5), of our thoughts (Rom2:16), our words (Matt.12:36-37), and our deeds (Rev.20:12), but the difference is that for believers, the true "judgment" of all that is sinful has already taken place - all our sinful thoughts, words, and deeds have been nailed to the cross of Christ and have been removed as an issue for salvation - we rely on the work of Christ. That is not to say that we are not to be careful about what we say, both in terms of avoiding the ill (Eph.5:4) and cleaving to the good (Col.4:6). Nor does it mean that we will not have to have the face to face accounting with our Lord for all our misdeeds, only that this evaluation will not, can not jeopardize our salvation in anyway.
One final observation: the speaking of harsh words against God is a particularly egregious offense in scripture (compare Jude 1:15 with 1:8), and is a significant characteristic both of antichrist (Dan.7:8; 7:20; 7:25; 8:25; 11:36; 2Thes.2:4; Rev.13:5-6; 16:14; 17:13-14; 19:19) and his most zealous supporters (1Tim.4:1ff; 2Tim.3:1ff; 2Pet.3:3-7; Jude 1:8; 1:15) - facts that account in part for our Lord's emphasizing of words in His reproach of these "vipers" in the context you ask about, Matthew 12:36-37, rather than of thoughts or deeds.
We Christians are not perfect, and we have a healthy fear of the Lord to help keep us on the right path, but we need also to remain encouraged that the blessings we have as a result of our faith in and faithfulness to Jesus Christ, kept secure for us in the heavens, are not to be compared with all the treasuries of this temporary world.
Please also see the following links which discusses the Last Judgment and the basis for unbeliever judgment:
The Last Judgment: Revelation 20:11-15
In Coming Tribulation part 6: Last Things: The Millennium and New Jerusalem, section VI, "The Last Judgment"