Shalom, I met someone that is a guer. When he did conversion and Tought that Yeshu was mashiaj, obviusly he never told anyone. 7 months later he realized this all was false. And he made tshuva. The question is , does he need to go to mikveh again or his conversion is valid.
Conversion requires the oversight and approval of a Beit DIn, a qualified Orthodox Jewish assemblage of 3 expert judges. One who was taught to believe ideas considered heretical in Jewish Law, was presumably not converted by a valid Beit Din. A member of his local Orthodox Rabbinate should be contacted to clarify the issue and convert him according to Jewish Law if necessary.