Do not swear falsely by My Name… (Leviticus 19:12)
Over the past three mitzvos, we have said: (1) Don’t steal sneakily; (2) don’t deny having someone else’s property since that’s stealing, too; and (3) certainly don’t swear falsely that you don’t have someone else’s property! This mitzvah continues the chain of mitzvos by telling us not to swear falsely about any matter whatsoever. (As we mentioned in the previous mitzvah, if a person swears falsely about possession of another’s property, he actually violates both Mitzvah #226 and Mitzvah #227.)
This oath only pertains to the speaker’s own actions. It can be about the past – that he did or did not do something – or it can be about the future – that the speaker commits to do or not to do something. This oath can only apply to things that a person can undertake voluntarily (e.g., “I will not fly on an airplane”). If he swears about a mitzvah, in which he is obligated in any event, his statement is irrelevant. (For example, you can’t swear not to keep Shabbos, since it’s not up to you. And if you swear to keep Shabbos? You have to anyway!)
God tells us outright why we are not to do this. The rest of our verse reads, “…doing so will desecrate My Name.” As we have said before, God is all about truth. We shouldn’t lie and if we do, God says, “Leave Me out of it!”
This mitzvah applies to both men and women in all times and places. It is subject of the Talmudic tractate Shevuos, starting with the first word on the first page (which is page 2a). It is codified in the Shulchan Aruch in Yoreh De’ah 236. This prohibition is #61 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #31 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.