I first learned this idea from Neil Gaiman's books of which I am a fan. According to his books (see American Gods) , once a god loses his flock of believers, he (or she) loses his powers and then he vanishes. Following this logic, any deity is interested in growing its number of believers. This would explain the missionary activities of various religions.
There might be an additional grim aspect of the gods created by men:
The believers who die in their name also increase their power. This fact could answer the following questions: Why did God let the Holocaust happen? Why are so many people ready to commit suicidal and murderous attacks in the name of Jihad?
In real life, don't believe seriously the above. Nevertheless, I used the ideas above in my books which I published on Amazon. In these books I used the ideas above to invent a theological paradox which says the following:
If various religions have created their gods who are maintained by the prayers and sacrifices of the believers, why shouldn't the atheist/agnostic community have their own god? He or she would be a weird god who thrives on the scientific and artistic progress of the people and would abhor any prayers. Therefore, the atheist god would prefer to be unknown and anonymous.
You can find my books on Amazon Kindle. Here is the list:
"The Freak God" (which will appear soon in an anthology of Digitalfiction.pub)
"New Gods, New Gravity" which is a prequel to "The Freak God"
"A New God in Athens" which is a sequel to "The Freak God"