Judgment would come on them for misleading the Lord"s people by falsely predicting peace when no peace was coming. [source][source][source]
There are two interpretations of the references to whitewashing, the literal and the metaphorical. The literal interpretation understands God to be saying that when the residents of Jerusalem built their walls (Heb. hayis, a flimsy partition) and houses, believing that they were secure, the false prophets supported their efforts by adding the whitewash. They should have warned them to prepare for coming judgment rather than helping them beautify the walls of their homes. The coming divine judgment would descend on Jerusalem like a rainstorm with hailstones and violent winds and destroy their beautifully whitewashed walls. [1][source]
The metaphorical interpretation, which most commentators took and which I prefer, understands God to be saying that these false prophets were putting a good front on the situation in Jerusalem, saying peace rather than judgment was coming. They were compounding Israel"s difficulties by hiding problems that needed to be exposed and corrected. Ezekiel was to tell them that invasion would come, like a rainstorm with hailstones and violent winds, and that their facade of a future for the people would then come crashing down (cf. Matthew 7:24-29). [source][source][source]
"The false prophets were compared to those who build an unsafe wall and cover up its defects. The untempered mortar [2] was actually whitewash, which is useless for strengthening insecure walls. Smooth words of false messengers hid from the people the actual seriousness of their spiritual condition. To daub with untempered mortar [3], in the metaphorical sense, is to flatter, to use hypocrisy. When the false prophets confirmed the people in their evil ways, by their approval they were whitewashing the flimsy spiritual structure of Israel." [3]6 [source]