Here are two possibilities:
* Classical Latin: thee-OH-loh-gus (emphasis on the second syllable)
* Ecclesiastical Latin: the-OH-loh-gus (emphasis on the first syllable)
In English, it is more common to use the word "theologian" which is pronounced thee-oh-LOH-jee-un.
If you encounter "theologus" in a specific context, it's best to look up its pronunciation in a dictionary or glossary for that particular context.