The church we attend has at least three or four armed guards plus another half dozen unarmed church security men at every service. It’s quite a large church with a fairly high profile, and I imagine there’s a lot going on behind the scenes that we don’t know about.
Even at our small church (typically, fewer than 150 people at 11:00 worship service), we have someone with a CCW permit patrolling the grounds and building during the service. There have been several highly publicized incidents (robberies, disruptions, etc.) during worship at churches in nearby towns.