A less than optimum situation may still be quite superior to other outcomes.
If one outcome is that an unoppposed attacker kills 26 and another outcome is that a poorly trained citizen kills the attacker and 1 innocent bystander, I'll take the latter.
This is wishful thinking. That poorly trained defender will probably be seated in the midst of the congregation -- not an effective position for engaging an active shooter entering the sanctuary. While most congregants are hiding behind pews/chairs, the untrained defender is sticking out like a sore thumb and will most likely become one of the first casualties of massacre.
An effective security team will choose where they position themselves with care so as to be able to quickly and effectively engage the active shooter as he approaches the church entrance. (Take the active shooter before he enters the sanctuary).
If the first team fails, then trained defenders within the sanctuary are prepared to engage the active shooter when he exists the foyer into the sanctuary. This is the right time take him up with effective fire from positions where no congregants will get caught in the crossfire.