Answer: Innovation is of two types: either it is an innovation that necessitates disbelief (Bid'ahtu Mukaffirah) or it is an innovation which is less than that (Bid'ahtu Ghayru Mukaffirah). In either case it is waajib (obligatory) upon us to call and give da'wah to those who affiliate to Islaam and have with them such innovations. And that we call them to the truth without assaulting or attacking what they are upon, except when we know that they have arrogantly turned away from accepting the truth. This is because Allaah the Most High said to the Prophet (صلى الله علیه وسلم):
"And do not insult those whom they (disbelievers) worship besides Allaah, lest they insult Allaah wrongfully without knowledge." [Sooraah al-An'aam: 108]
So we firstly call them to the truth, by clarifying the truth and making it clear with evidences. And the truth is accepted by every individual who has a sound Fitrah (natural disposition to the truth). Then if rejection and arrogance is found in them, we make clear their falsehood, without falling into argumentation with them. Such an affair is obligatory.
As for making Hajr (abandoning) them, then this depends on the type of innovation. If the person is upon Bid'ahtu Mukaffirah, then it is obligatory to abandon him. If however he is upon something less than that, then we halt and refrain from making Hajr, until we know that in making Hajr of him there is a Maslahaa (benefit) then we do it and if there is no benefit, then we refrain from it. This is because the A'sl (fundamental principle) with regards to the believer; is that it is Haraam to make Hajr of him, based upon what the Prophet (صلى الله علیه وسلم) said:
"It is not allowed for a believer to make Hajr of his brother for more than three days." [Muslim]
So as long as there is no benefit in the Hajr, then it is Haraam to make Hajr of every believer, even if he is a Faasiq (sinner, wrongdoer). If however there is benefit in the Hajr, then we abandon him. This is because Hajr is a medicine. As for where there is no benefit or it only increases him in sins and haughtiness, then where there is no benefit in an affair then leaving that thing is beneficial in itself.
If however, a person was to say: 'This is rejected because the Prophet (صلى الله علیه وسلم) made Hajr of K'ab bin Maalik and his two companions when they remained behind from the battle of Tabook.'
So the answer is: This occurred from the Prophet (صلى الله علیه وسلم) and he ordered that Hajr be made of them. This was because in making hajr of them, there was a great benefit. For indeed it only strengthened and firmly established them and increased their adherence to what they were upon. To such an extent, that a letter came to K'ab bin Maalik from the king of Ghasaan, where he said: 'It has indeed reached me that your companion (meaning the Messenger - sal-Allahu 'alayhe wa sallam) has rendered you useless and indeed you are not from a place of disgrace and humiliation, so join us and we will console you and comfort you.' So he took the letter and burnt it in a furnace and said: 'This is indeed a test.' And he was truthful, as it was indeed a test and a trial. If it wasn't for the fact that his Eemaan (faith) was well grounded, he would have been happy with this letter and would have gone to the king to be from amongst his children. Look at his discerning wisdom (may Allaah be pleased with him), wherein he went with the letter and burnt it in the furnace. This is because he feared that if he had kept it, then he would of thought about it a second time. Then Shaitaan would have lead him astray and he would have gone to the king of Ghasaan.
So with regards to these three, there was a great benefit in making Hajr of them. Then the result (which cannot be matched or be equated with anything) was that Allaah sent a verse regarding their situation, which is now read till the Day of Judgment:
"And (He forgave also) the three (who did not join the Tabook expedition), till for them the earth was straitened and their ownselves were straitened to them, and they perceived that there is no fleeing from Allah, and no refuge but with Him. Then, He forgave them, that they might beg for his pardon. Verily, Allah is the One Who forgives and accepts repentance, Most Merciful." [Soorah at-Tawbah: 118]
Then He said:
"O you who believe! Fear Allah and be with those who are truthful." [Soorah at-Tawbah: 119]
Therefore Allah sent down this verse regarding them, because they were truthful.