89% of evangelical church parking lots contain one of these signs. The signs are never positioned so that you see them while you’re driving into the lot. They’re placed so you can only see them as you’re driving away.
The implication is that you were not in the mission field while you were on church grounds, but in a cozy bubble away from pesky non-believers and moral miscreants.
Some churches post the sign above the church exit. Maybe they’re reminding you that even the parking lot is a battlefield.
Other churches post the sign on your way out of the sanctuary, maybe as a heads-up that there could be some spiritual warfare in the narthex.
To my knowledge no one has ever posted one of these signs on the way into a church building, even though a lot of the time you are safer away from a church than inside of it.