
The word "infected" is important. It keeps coming up again and again. A great work of art infects its audience like a benign disease. They're mostly incapable of resisting the infection and they'll likely spread it to others. That's how art contributes to the spread of great ideas.
Some people through malice or stupidity are resistent to artistic infection and Tolstoy has no use for them. For Tolstoy critics fall into this category. I like critics myself. They're often wrong but they get useful arguments started and stimulate the market for art.
Examples of art he approved of were Dickens "Christmas Carol" and "David Copperfield" and Hugo's "Les Miserables."

Sincerity and earnestness is precicely what a lot of modern media lacks. It's a measure of the greatness of some writers that they dare to voice great truths even though the truths are intuitive and are often difficult to express with words.