This is something that was left out of the New God's Handbook.
Loss of Faith
If you spend less than 1 Belief per 10 followers to create Miracles that directly help them, you create loss of faith. Your followers believe you have abandoned them or are ignoring them. This year, they produce half their normal Belief (see “Gain Belief,” below).
Followers can suffer from loss of faith many years in a row—as long as you don’t spend enough Belief, they lose their faith. After two consecutive years in which the followers suffer from loss of faith, you lose 10% of them. They leave for greener pastures: for gods who care about them. If another year passes in which the followers have lost their faith, another 10% leave, and so forth. You lose these followers before you gain Belief for the year.
It should be at the end of Chapter 5.