What does Edwards mean by “keep up the form of religion”?

I think it means continuing the religion and making more people understand this religion in order to "keep it going".

Explanation:

Hope this helps. ;)

"valley of salt tears" and "dark, deep and foggy" are two images used to establish a mood of uncertainty and regret. this imagery is made up of sensory appeals to brings up   theological concerns of sin and retribution.

they conjure up the human   uncertainty about the divine and its relationship to the world.