Hawkeye

 

Also known as Clint Barton.

There isn't much to say on religion regarding Hawkeye. The only instance through the comics that there is any mention on his religious affiliations is when he pretended to be dead and had a Catholic funeral.

This to me makes it safe to assume he is Catholic, but from the lack of religion mentioned surrounding Clint, it's also safe to assume he is non-practicing. 

He seems to have good knowledge on religion as he can contribute to conversation regarding it, an example is when he refered to Loki as a "Norse God."