'Insincere' in the Bible

Like a [common] clay vessel covered with the silver dross [making it appear silver when it has no real value]Are burning lips [murmuring manipulative words] and a wicked heart.

The refining pot is for silver and the furnace for gold [to separate the impurities of the metal],And each is tested by the praise given to him [and his response to it, whether humble or proud].

Some of those who are [spiritually] wise and have insight will fall [as martyrs] in order to refine, to purge and to make those among God’s people pure, until the end time; because it is yet to come at the time appointed [by God].