Question

the poem "snake" has a religious and social theme

Answer 1

Social as in societal class structure of wealthy, middle class, poor dominating existence. The man owns the trough; he goes to fill up his pitcher; but a "majestic" snake drinks water from the stone trough first. This doesn't bother the man, and he actually finds the snake not threatening, but honors him as though the snake was of the higher class, "honored" still: 

"......That he should seek my hospitality 
From out the dark door of the secret earth." 

And when the thankless snake crawls through a black hole, the man is angered, and throws a log at the snake. It doesn't hurt it except a piece of his tail. And the man immediately feels regret (religious theme, read on the ending of the poem): 

" And immediately I regretted it. 
I thought how paltry, how vulgar, what a mean act! 
I despised myself and the voices of my accursed human education 

And I thought of the albatross 
And I wished he would come back, my snake. 

For he seemed to me again like a king, 
Like a king in exile, uncrowned in the underworld, 
Now due to be crowned again 

And so, I missed my chance with one of the lords 
Of life. 
And I have something to expiate: 
A pettiness." 

Religious Theme: "And I have something to expiate", the man seeks religious atonement for his pettiness in hurting the snake who had not harmed him in anyway, the very same snake he holds in high regards and admires. 

He wants to repent his pettiness