Breaking Point

Security. 
As a function, it can be as complex as 24/7 coverage with rotating shifts of highly-trained and motivated individuals with the authority to solve nearly any problem that arises with supporting functions such as communications, video surveillance, firearms training, certification, and control, with strict rules and regulations to remain compliant within. 

It can be as simple as the lock on the door, or as complex as a fully staffed, full time, security force, and in between is a sea of options and confusion. 

Here are some realities at both ends of the security options spectrum:

Minimum: A lock on the door, except when the church is open and someone keeps an eye on those who enter. This is where many churches are. Confidence in the Saviour is well-placed and is the best security one can have. However, the Lord instructed His people to build walls while keeping one hand on their weapons. Often, they were sent to war, and they were also told to be prepared for war. 

Prepare The Horse literally means to get everything ready to go to war, but leave your safety to the Lord. Being prepared means different things to different people and is defined by your expectations.

The probability of any particular crime or serious incident happening at a certain location is usually in line with the crime statistics of the surrounding area. While the chances of a mass shooting are likely to be low, the chances of domestic arguments might be high. Domestic arguments can be, and often are, violent. Being emotionally volatile, any weapon brought into the problem can make it a traumatic incident.  

The church is not the home. People act differently at home than they do in church. They shouldn't, but they do. Most people can resist bringing their home issues into the church, but, history shows that it does happen. 

The "minimally" prepared church or business will probably have someone in the church that can "step in" or mediate any arguments or disputes, up to a point. There is a tipping point in nearly every emotional dispute where a higher level of experience and ability needs to take over. This is where the minimally prepared church will have issues, and the chance of violence occuring increases every passing second without intervention. 

Medium: Hiring a local police officer or sheriff's deputy for a few hours a week. This can solve nearly all of the issues surrounding the legal requirements and authority limits of anyone in a security position. The officer has authority, jurisdiction, the equipment, the training, and the local legal system supporting them. Insurance requirements may need to be met, but the training and jurisdiction will most likely take care of any liability issues if needed. The visibility alone of the uniformed officer and a marked patrol vehicle will deter most criminals.

Having worked as a full-time patrol officer on shift work and being hired as part-time security for quite a few churches and other houses of worship, it was a nice bit of spending money for just a few hours a week. However, the full-time job and family matters came first, which meant that there would be days with no officer present. This problem CAN be worked around with some creativity. Issues in this category are contracts and agreements, pay and taxes, scheduling, and communication with church leadership for detailed instructions. 
 
You could also outsource your security needs with an already-formed company. The downsides to this are obvious and plenteous. Contracts, fees, licensing, and liability deferment are just a few. One of the largest security companies in the nation that provides services to governmment buildings and properties provides "armed security guards". However, during an evaluation of a large government 3-letter organization's DC-based operations center, it was discovered that the already-bought-and-paid-for contract specifically stated that the security guards were, "under no circumstances" to draw their weapons while on duty. This halted our evaluation of the plan and the contract was renegotiated. 

Maximum: Forming a security company and hiring people to work as paid security officers, dedicated to your organization or location. This gives your organization almost complete control over the security of your church and members. 
The issues surrounding this option are many. In most states, a security company must have licensing, certification, bonding, insurance, extensive documented and scheduled training and testing by state agencies, as well as evaluations at regular intervals for compliance. Further, any agent of your organization that acts outside the scope of their work, can bring enormous liability issues on your church and membership, transferring legal fees and penalties to your organization.  The cost of upkeep for even a modest security force can be in the tens of thousands per year. 

Somewhere in the middle, but comfortable. 
This is where most churches want to be. Keeping expenses down, morale and sense of safety and comfort is up, and a few capable members have organized an effort to keep a watchful eye on the door and congregation. 
In some states, where second amendment restrictions abound, the only choices are to hire an officer or pay for a security company. 
In areas where concealed or open carry of firearms is allowed, the list of options changes. For the better, I believe. 

Regularly attending and bona fide members who have the capability/ies, opportunity, motivation, and discretion, would be prime candidates for a security team. Most important among these is discretion.  Other helpful and bonus characteristics would be experience in the security field, positive experience in confrontations, a degree of self-defense training, and a little range time with firearms wouldn't be bad either.

Three of the four basic requirements, capability, opportunity, and motivation will be (or should be) immediately apparent. A little time and familiarity with a person will give some insight to their capabilities and their motivation, and the opportunity will have to be discussed, but only after bringing the subject of security up. While any team leader or member of management wants those three qualities in a person they work with, if the candidate does not display discretion, no amount of the other three characteristics will be enough to balance them. 

Discretion should improve with experience, but this is not the time for training. Although incidents of one type or another occur frequently, this is not the environment for fledgling security guards to "work their skills" on the members and visitors. 

There are more restrictions, requirements, and considerations that go into this decision tree, but in general, having volunteers that are experienced, capable, motivated, and available, and conduct their personal interactions with others with discretion, can benefit your church greatly, in more ways than one.