Is one considered an infidel who is ignorant of the ruling of something that is known to be an indispensable part of the religion, but does not deem unlawful what is lawful, nor deem lawful what is unlawful, such as saying "I don't know" when asked about adultery?

One who denies the unlawfulness of zina (fornication/adultery), or doubts it, or hesitates concerning its prohibition, while living in Muslim lands and not ignorant of this matter, is a disbeliever, an apostate. This is because the prohibition of zina is known by necessity, and no doubt can assail it. However, one who is a recent convert from disbelief, or grew up in a distant desert and the prohibition of zina was hidden from him, is not declared a disbeliever except after clarification and the establishment of proof. In conclusion: declaring a specific individual a disbeliever is a matter for the people of knowledge and the judiciary.