The professor is a committed ideologist who never wavers in his political beliefs.

ideologist 是由名词 ideology（意识形态）加上后缀 -ist 构成的名词。-ist 表示"从事某种活动的人"或"某种主义的支持者"，所以 ideologist 指的是"意识形态学者"或"意识形态专家"。