It means that such a person must believe in the essential requirements of the Islamic faith, and he should not do anything against the spirit of the faith. In the same book we come across this statement:
“That is to say, it is an agreed view of the Islamic scholars that a person who violates the essential requirements of Islam is a disbeliever, though he may be a follower of the Qiblah and he may spend his entire life in performing the acts of obedience, as has been stated in Sharah-E- Tahreer by Imam Ibnul Hammam.”
The books of Islamic belief, law and principles are full of clear directives and details on this point.
(Tamheed Ul Iman by Ala Hazrat radi allahu anhu)