Benjaminn, I am going to disagree. Visible presence IS a deterrent-we all see this every day. Think of the cop cars parked on the side of the road-even unoccupied, people STILL slow down. Seeing security there-and it could simply be blazers-having a VISIBLE presence deters criminals. That being said-someone intent on doing someone harm WILL attempt it-security or no. Having a strong visual presence can deter your average criminal-you are hardening your target. Statistics prove out that simple visual queues can deter most criminals-many, many people put up security alarm system signs on their lawns-and dont have an alarm system. Some even go so far as to purchase fake window breach alarms, in case the criminal decides to look for himself. HE doesnt know its fake-but he sees it, and will hopefully look for an easier target.
A visible security force, a good, MONITORED alarm system, and, if you want, an invisible backup force are the way to go. But, again, this is likely out of the realm of the budget for the church. At the very least, I would look into a panic button to the local PD.