Is one who denies adult suckling or mocks it considered an apostate or one who denies a matter known to be an indispensable part of the religion?

Denial of adult breastfeeding that does not establish mahramiyyah (prohibition of marriage) is not blameworthy, as it is the view of the majority of scholars. As for denying a hadith (definition) out of ignorance, it is not considered a denial of what is known by necessity in the religion. Mocking an authentic hadith after knowing its authenticity is dangerous, and there is fear of apostasy for the one who does so. For someone who was ignorant of the hadith's authenticity, we hope that they have not fallen into apostasy.