In the context that I used it, the popcorn Christians are folks that are good Christian folks normally, but have dedicated little to no time on research on candidates past word of mouth information. They likely don't know Huck's record versus Fred's. Odds are they have never heard of half of the candidate that are running, and if they have, they couldn't tell you one thing about them.
The term is one I self made and is basically derived from watching football fans watching bands play at halftime. To me the best part of the game is the band's performance at halftime, but to the popcorn eaters, it is just time to go get a coke and pee. The may occasionally clap to be polite, but one can tell there is little attention to detail. Vice versa could be said for some people during the football game itself of course. I have widened the meaning to include all people that really don't have a caring interest in something that requires judgment. Voting should be an serious issue, perhaps us FReepers take that to an extreme, but popcorn voters tend to bug me enough that I made my own lexicon for it.