-Least religious district - WA-7
Definitely not that. Major urban centers always have a significant population of immigrants, minorities and poor whites who tend to be religious. The suburban districts there are probably far less so.
Actually Seattle is an odd city, the population is two-third White, and its largest minority group is Asian which tends to be rather nonreligious. Tacoma has a larger minority population actually. Next door, Bellevue is a suburb with a large immigrant population, but is still mostly nonreligious. Overall, the Seattle area lacks much religious following although in some exurbs you might find some very religious people. My district (the 8th) is probably the least religious state with a Republican in office, mostly because people here are quite fiscally conservative.