Imaam Ibnul Qayyim (rahimahullaah) said:
As for those who came after the Sahaabah (i.e. us), among them is that one who was brought up in Islaam but knows not what is in opposition to it. Therefore, some of the details of the path of the believers and that of the criminals became confusing to him, for indeed confusion occurs due to the weakness of one’s knowledge regarding either paths or one of them, as Umar Ibnul Khattaab (radiyallaahu-anhu) said:
”The ties of Islaam will be demolished one after the other when a people brought up in Islaam know not what Jaahiliyyah (Pre-Islamic Ignorance) is.”
This (statement indicates) Umar’s excellent knowledge. And if you do not know what pre-Islamic ignorance and its rulings are, then it is all that is in opposition to what the Messenger came with-all (affairs) that are in opposition to the Messenger is tantamount to ignorance.
And there is that one who is neither acquainted with the path of the criminals nor has it been made clear to him, or he has doubts and thus thinks that some of their ways are from the ways of the believers. This has occurred in this Ummah in many affairs pertaining to Aqeedah, Knowledge and Action from the path of the criminals, the disbelievers and the enemies of the Messengers, (which) was brought into the path of the believers by the one who does not know the (detailed distinction between) the path of believers and the path of the criminals, so he calls to (this path of the criminals); excommunicates the one who opposes him and declares lawful that which Allaah and His Messenger made unlawful, just as what has occurred with many of the people of bidah, such as the Jahmiyyah, the Qadariyyah, the Khawaarij, the Rawaafid and their likes, among those who initiated an innovation, called to it and excommunicated those who opposed it.
[Source: Al-Fawaa’d pages 167-180; abridged and slightly paraphrased]