It is reported from Sufyān Al-Thawrī – Allah have mercy on him – that he said:

A person who sits with a heretic (an adherent of bid’ah) will not escape one of three things:

Either he will become a trial (fitnah) for others, or some deviation will occur in his heart and he will slip and be cast into the Fire by Allah, or he will say to himself, ‘By Allah, I don’t care what they say, I am confident about myself;’ but whoever feels secure from Allah about his religion even for the blinking of an eye, Allah will take his religion away from him.

Ibn Waddāḥ, Al-Bida’ p125.