Chapter five of The Great Gatsby by F. Scott Fitzgerald begins with the rather surprising image of Gatsby's mansion as Nick comes home late one night:
Two o'clock and the whole corner of the peninsula was blazing with light which fell unreal on the shrubbery and made thin elongating glints upon the roadside wires. Turning a corner I saw that it was Gatsby's house, lit from tower to cellar.
Finally, Nick goes to bed while Gatsby's "house blazed gaudily on." From the beginning we have the obvious images referencing light, and the imagery continues the next day, the day Daisy comes to Nick's for tea.
It is a rather rainy day by the time Daisy arrives, and it is clear that Gatsby is tense and quite nervous; in fact, he is so on edge that he just disappears just before she arrives. He does eventually come in from the rain and the meeting between him and Daisy is awkward and stilted. Nick has to give Gatsby a little pep talk before Nick leaves.
After a while, the rain stops and Nick goes back inside; he can hardly believe what he sees:
there was a change in Gatsby that was simply confounding. He literally glowed; without a word or a gesture of exultation a new well-being radiated from him and filled the little room.
This third reference to light (glowed, radiated, filled) comes right before the passage you cite in your question. Nick announces that it has stopped raining, and what Gatsby says next is full of more light references.
When he [Gatsby] realized what I was talking about, that there were twinkle-bells of sunshine in the room, he smiled like a weather man, like an ecstatic patron of recurrent light, and repeated the news to Daisy. "What do you think of that? It's stopped raining."
This passage contains two similes which are also light-related. First, Fitzgerald says Gatsby "smiled like a weatherman" as sunshine fills the room. This comparison suggests that, just as a weatherman is happy to give a forecast of sunshine (as opposed to rain), Gatsby is thrilled with the sunshine--both the literal sunshine and the figurative sunshine which Daisy has brought him.
The second simile is the one to which you refer, and it seems evident that we are to take it both literally and figuratively, like the weatherman comparison.
Ecstatic simply means "feeling or expressing overwhelming happiness or joyful excitement." A patron is a supporter or a customer. Though Gatsby has plenty of electric lighting in his home, electricity was not common in every home in America at this time. We are so used to having electric lighting that it would be hard to imagine the awe one might feel when his house was lit up, so to speak, for the first time. His face would be full of awe and wonder at the seeming miracle which has happened right before his eyes.
In this simile then, Fitzgerald is comparing Gatsby's radiant happiness to that of a customer who has just gotten electricity in his home for the first time. Literally, he is as happy as a homeowner who just received the personal miracle of light, and figuratively he is overjoyed at Daisy's shining presence in his life.
The other imagery in the chapter is not suggestive of any religious overtones, but the chapter is filled with several kinds of light imagery, which supports this reading of the words "ecstatic patron."