Gallup isn't good at polling stuff like this. The NORC's GSS is where you want to go.

E.g. How often do you attend a religious service?

Once a week has been falling, Never has been rising - especially among the young.

How religious is the respondent? "Very religious" has been under 20% since 1998. "Not religious" has been rising almost linearly since 2006 (for all groups except those over 65).

There's lots of great info about what Americans really think about questions of the day there.