Is it permissible not to greet a person of innovation (bid'ah) or one who declares a Muslim an infidel for abandoning prayer, and to boycott a Muslim instead of advising him, especially if that person holds to Irja' (the doctrine that faith is merely outward confession, and sins do not diminish it)?

Greeting the people of innovation (Ahl al-Bid'ah) varies according to their innovation. If their innovation is one that leads to disbelief (mukaffirah), then they should not be greeted with peace. If their innovation is not one that leads to disbelief, then the interest (maslahah) should be considered. If there is a benefit in not greeting them with peace (such as deterring them from their innovation), then it is obligatory to refrain from greeting them. However, if shunning them would only increase their corruption, then they should not be shunned. It is permissible to refrain from returning the greeting of one who commits a forbidden act, as a deterrent and a rebuke to them.