What are the criteria of disbelief by which imitation of disbelievers transforms from a sin into disbelief?

Mere imitation of disbelievers is not an act of apostasy that takes one out of the fold of Islam, unless the imitation is in disbelief itself. Sheikh al-Islam Ibn Taymiyyah mentioned that the Hadith "Whoever imitates a people is one of them" can be interpreted in two ways: either absolute imitation which necessitates disbelief, or that the imitator becomes one of them in the shared aspect in which he imitated them. In any case, the Hadith indicates the prohibition of imitating them in what is specific to them. The scholar Ibn Uthaymeen clarified that the meaning of "he is one of them" refers to the outward appearance and demeanor that resembles theirs, and that outward imitation of them may lead to inner imitation.