Is the Baptism of One Who Rejects a Teaching Valid?

Yes. The baptisms by heretics and schismatics are considered perfectly valid (Council of Trent, session VII, canon IV). The minimum faith required for adult baptism would be a belief in the Trinity and a willingness to be a disciple of Jesus. They are not required to have a fully developed understanding of the Faith.

“The faith required for baptism is not a perfect and mature faith but a beginning that is called to develop” (Catechism 1253).