He Is Not From Us!

A believer must always be careful to avoid anything that distances him from Allāh, or from the way of the believers, or from what Allāh is pleased with, especially matters that the Messenger of Allāh ﷺ has clearly forbidden. But how serious is it when the warning comes in the words: “لَيْسَ مِنَّا - He is not from us”?