Is learning the arts of non-Muslims or Muslims in lands known for innovations (bid'ah) and rejection, such as Persian calligraphy or schools of Ottoman Arabic calligraphy, considered an imitation of them? And is consulting original references in this regard a sin due to the possibility of them containing their beliefs?

Imitating innovators or non-Muslims is prohibited regarding what is specific to them, such that anyone who sees it would think that its practitioner is one of them. The criterion for imitating disbelievers is that a person does something specific to them. As for what is common between Muslims and disbelievers, it is not imitation. Therefore, whatever has become widespread among people and does not distinguish disbelievers or transgressors from others, it does not fall under the prohibition, even if its origin was taken from disbelievers. Calligraphy, for example, is used by Muslims and others. Imitating disbelievers is in what is specific to them, not in their language, or in adopting their crafts and experiences. It is better for a Muslim to avoid speaking and writing in languages other than Arabic when there is no need. There is no harm in consulting original sources that may contain their beliefs, if a student needs them and feels secure from being influenced by them.