The Big Lie
Did Korach and his followers really think that Egypt, the house of bondage was for them a land flowing with milk and honey! Hashem forgave Bnei Yisrael for the Golden Calf which was an assault on Him but not for the sin of the Meraglim, which was an assault on Eretz Yisrael. When Datan and Aviram apply the term Land Flowing with Milk and Honey to Egypt the rebellion of Korach, like the sin of the Meraglim became an unforgivable offense. Korach's rebellion, on the other hand, was an assault on God and Moshe's leadership (similar to the Golden Calf).
Both Korach's claim and that of Datan and Aviram were outright lies. They weren't simply people who lied there were liars.
Delivered at the OU Israel Center, June 25, 2019 (22 Sivan 5779)