What is the ruling on taking a specific preacher as a fundamental reference for understanding the religion, and detesting everyone who disagrees with him, and what is the ruling on that if he is merely a student of knowledge following the methodology of a particular theologian?

It is permissible to benefit from preachers and students of knowledge, even if they are not deeply learned, regarding what is known to be true. It is permissible for a layperson to benefit from those who have been influenced by scholastic theology (Kalam), without imitating them in their doctrinal deviations, if no others are available. However, complete imitation of them must be avoided. It is forbidden to be fanatically devoted to a person to the extent that one allies or disallies on their behalf. Rather, one should love all Muslims and hate disobedient individuals only to the extent of their disobedience.