What a great concept -- mix-up the genres of Westerns and Sci-Fi. Harrison Ford did it previously in the original Star Wars (aka A New Hope), don't you think!
Though the potential was there, once again the Summer blockbusters are just not quite getting there. Although huge amounts of money are flowing into Hollywood coffers this years, the quality of the films seems to be off a bit. [But let me say that Jon Favreau's meteoric rise has been well-deserved. The directing is good; I think the writing is probably weak -- never really got beyond the original cool idea with several formulas tossed in.]
Some religious theme could have been anticipated as the town's name was Absolution.
There is an interesting character in the film -- a preacher / doctor / posse member. Though there is a cross close to the pulpit in an area that also served as a surgery suite, the "theology" seemed weak. In fact, it was strongly the traditional understanding of religion as doing right, morality alone, personal accomplishment judgment. What exactly was God's function in this character's theology? And how did he mesh it with ALIENS?
Sadly we never found out as he tried to do good, and got stepped on by one of those pesky demon Aliens!
We wait for a good religious reflection in current cinema. Dare I say that the closest we have gotten so far was in Easy A among the mainstream films?