Shaykh Rabee (hafithahullah) said, “It is well known from Ahlu Sunnah that whoever fell into an affair necessitating unbelief that takfir is not performed until the proof is established upon him.
As for the one who falls into an innovation, then he is of categories.
The first category: Ahlu Bidah such as the Rawafid, the Khawarij, the Jahmiyyah, the Qadariyyah, the Mu’tazilah, the Grave-worshipping Sufiyyah, the Murji’ah, and whoever is connected to them [in our times] such as Al-Ikhwani [Muslimoon], At-Tabligh and those similar to them. For these innovations the Salaf did not make it a condition that the proof be established [upon them] due to the fact that the ruling upon them is [in accordance to] the innovations [they openly act upon, call to/promote and relentlessly defend in the face of overwhelming evidences pointing out the incorrectness of their innovations].
So regarding the Rafidi, it is said about him, “Innovator (mubtadi).”
As for the Khariji, it is said about him, “Innovator”, and so on, regardless of whether the proof is established upon him or not.
The second category: The one who is from Ahlu Sunnah and then he fell into a clear-cut innovation such as the saying that the Quran is created, or the denial of the Pre-Decree, or he has the views of the Khawarij or other than that. Then upon this person tabdi is performed, i.e. he is declared an innovator – and this was the behavior of the Salaf.
An example of this is what is narrated from Ibn Umar (radiyallaahu ‘anhumaa), when he was asked regarding the Qadariyyah, he answered, “When you meet them, inform them that I am free from them, and they are free from me.”
The third category: Someone who was from Ahlu Sunnah, well known for seeking the truth but he fell into a obscure [or a bidah which is difficult to determine/detect].
So if this person dies, then it is not permitted to make tabdi of him, rather he is mentioned with good. But if he is alive, he is advised [with patients and good manners privately several times] and the truth is made clear to him and one does not hasten to make tabdi of him, but if he persists [upon innovation after he is advised with the truth repeatedly with good manners], then he is declared an innovator.
Shaykhul-Islam Ibn Taymiyyah stated, “There are many from the Mujtahidin (scholars of ijtihad) of the Salaf and of later times who spoke with and acted upon innovation whilst not knowing it to be an innovation. This was either due to weak ahadith that they thought were authentic; or Quranic verses they understood in an incorrect manner; or due to an opinion one would hold in an issue because the texts had not reached him.
So if a man fears his Lord to the best of his ability, he enters into the saying of Allah: “Our Lord, do not take us to account if we are forgetful or mistaken.” And there occurs in a hadith that Allah stated [in response], “I have done so.” And this matter has been explained in another place.” 
Nevertheless the point is that it is not permitted to place unrestricted conditions regarding the establishment of proofs upon Ahlu Bidah in general nor negate these conditions unrestrictedly [in general] – and the affair is as I have mentioned [sometimes you need to establish evidence upon a person to declare them an innovator and sometimes you don’t need to establish evidence upon a person to declare them an innovator].
Thus my advice to the students of knowledge is that they hold fast on to the Book and Sunnah and that they are disciplined and grounded upon the manhaj (methodology) of the Salaf in every aspect of their religion – and specifically in the arena of takfir, tafsiq and tabdi so that there is not excessive disputing and argumentation in these matters.
I particularly advise and encourage the Salafi youth that they abandon those affairs that sow malice, rancor, differing and splitting – affairs that are hated by Allah and which he warned against – affairs that the noble Messenger (salallaahu ‘alaihi wassallam), the Companions and the Righteous Salaf warned against.
Also [I advise] the Salafi youth to strive to spread those affairs that lead to love, affection and brotherhood between themselves – those affairs that are loved by Allah (the Most High) and His Messenger (salallaahu ‘alaihi wassallam).
May the salutations and peace of Allsh be upon our Prophet, his family and his Companions.”
Ref: sahab.net in Arabic via salafitalk.net in an abridged translated form for the sake of brevity.
1. Takfir is to declare a person to be an unbeliever outside the fold of Islam.
2. Tabdi is to declare a Muslim to be an innovator and therefore outside the sphere of Ahlu Sunnah Wal Jamaah.
3. Reported by Muslim.
4. Though such a person is mentioned with good, nevertheless his mistaken and erroneous remarks are clarified by the scholars so that the Muslims are not misled, as Shaykh Rabee has mentioned in numerous other statements.
5. Ma’ārij al-Wusūl, p. 43.
6. Tafsiq is to declare a Muslim to be an open rebellious sinner.
Posted by Abdul Kareem Ibn Ozzie