Then they cannot test theory to practice, and thus fall into traps of idealism. Theory informs practice, and practice confirms or denies theory. This cycle is repeated over and over, sharpening theory and practice against each other. Without the other, each is dull. Take it from Mao:

The method of studying the social sciences exclusively from the book is likewise extremely dangerous and may even lead one onto the road of counter-revolution. Clear proof of this is provided by the fact that whole batches of Chinese Communists who confined themselves to books in their study of the social sciences have turned into counter-revolutionaries. When we say Marxism is correct, it is certainly not because Marx was a “prophet” but because his theory has been proved correct in our practice and in our struggle. We need Marxism in our struggle. In our acceptance of his theory no such formalization of mystical notion as that of “prophecy” ever enters our minds. Many who have read Marxist books have become renegades from the revolution, whereas illiterate workers often grasp Marxism very well. Of course we should study Marxist books, but this study must be integrated with our country’s actual conditions. We need books, but we must overcome book worship, which is divorced from the actual situation.

Oppose Book Worship (emphasis mine)

That’s why it’s best to join a party, even if that means starting a book club as the basis and growing organically from there.