Religion made occasional appearances as part of the culture of Hill Valley both in its institutionalized practice and in the moral behavior of its citizens. The form of the practice of religion, however, has changed throughout its history.
On Sunday morning, as Marty faced the possibility of his own death in a gunfight the next day, a carriage passed him, apparently on its way to church or an outdoor revival meeting, and a man patted his Bible and told him, “We’ll be praying for you.”
As Doc and Marty were about the leave the saloon to catch the train, one old-timer offered a final toast, "To the future", and another old-timer answered "Amen!", to which Doc repeated "Amen!" and drank a shot of whiskey.
While he was at Irving "Kid" Tannen's speakeasy El Kid, Marty asked Trixie Trotter about requesting several songs, which have not been conceived this early in time. The first was Sister Christian, to which Trixie responded that she doesn't sing religious tunes. The second was Stairway to Heaven, to which Trixie responded that if Marty wanted hymns, to go to a church.
Near the end of his letter that he prophetically wrote one week before his death, Doc composed his heartfelt, final words to Marty, "... I now say farewell and wish you Godspeed."
Although Doc Brown was a scientist, he chose the birth of Christ as one of the two most significant events in history that people would want to visit if they had a time machine; the other being the signing of the Declaration of Independence.
Biff’s selfishness and lack of morality set the standard for whole town of “Hell Valley”. Marty asked Doc whether they were in Hell, and Doc confirmed that it was Hill Valley although Hell could not be much worse.
Several Hare Krishnas marched down the street in Courthouse Square as Marty looked toward the Cafe 80s. Apparently, South and East Asian immigrants brought their religion as well as their culture to the Hill Valley area.