Over the weekend, cutting through the Albertson's parking lot on the way to Home Depot, I saw a minivan in the loading lane, with just such a sticker on the side window. I parked and walked over to the van, whose driver was texting while waiting for someone to come out of the store.
Keeping a safe distance from the driver's window so as not to appear threatening (cause you know how intimidating middle age white guys can be), I approached the vehicle. The woman looked up from her phone, looked me over, determined I was harmless, and rolled down the window.
Turns out the logo is from Fellowship of the Sword, a group that it not a church, but operates revival retreats in a half dozen states, including one in the Texas Hill Country. The organization is based in West Fort Worth/White Settlement. I remember when I attended a singles group at Gateway a few years back, several people there were very excited about involvement/attending FTS activities.
Google results appear to indicate that some regard FTS as 'cult-like', though it's not clear to me whether there's any basis to that or not. I know a woman who attended a Methodist (very mainstream) retreat many years ago that had some elements I regarded as cultist, so it may just be a subjective matter of perspective. And apparently ABC News thinks all defenders of traditional marriage are Westboro Baptist sympathizers.
Anyway, the mystery is no more.