What makes a book a classic is simple: it lives (continues to sell and be read) beyond its normal expected years.  Well, long beyond its expected lifetime.

There are three ways for a book to live long enough to become a classic.  Its readers keep it alive.  Its teachers, scholars and critics keep it alive.  And other authors keep it alive.

After 50 years, J R R Tolkien's Lord of the Rings continues to be a big seller (independent of Peter Jackson's visualization).  Readers keep it alive.

After 60 years, Edith Hamilton's Mythology continues to sell well.  Teachers keep it alive.

After 400 years, Shakespeare is still King.  Samuel Johnson brought him back from the dead.

The Bible and the Qu'ran are kept alive by zeal, profit and guilt.  Which by NO means deprives them of their rightful Classic status.