Who is John? That is, what is the history here? How does Granny Weatherall feel about him?
John is Granny's husband who had died many years previously. On the day she was jilted, her world was collapsing, but John's "caught her under the breast, she had not fallen." John rescued her and saved her from total despair. He said that he would kill George for her, but she said "for my sake leave something to God." When John says "Now, Ellen, you must believe what I tell you..." we do not know what John said, but we can suppose that he professes his love to her and asks her to marry him.
Granny loves John as a wife and mother of his children, but she still thinks of George in a romantic way. Near her death it is George that she wishes to see after 60 years. It is also George who is tied to her lack of spirituality because of her feelings of guilt. Her bitterness results as a wasted fruit which could have been the love they shared
John is the man Granny eventually married and had children with after she was jilted by George. She speaks of him fondly, and they must have shared a deep closeness, because as she is thinking with satisfaction about the children and horses and negroes she cared for in her life, she remembers, "John would see that in a minute, that would be something he could understand, she wouldn't have to explain anything!" John died a long time ago, and Granny realizes that now "all (her) children (are) older than their father." She muses at the strangeness of the idea that time stopped for him when he died, but it went on for her, and she has changed - "she used to think of him as a man, but...he would be a child beside her if she saw him now."