Someone determined to cause harm to a specific individual will make all attempts to do so-they arent right in their head, so I do agree that a visible security presence may not exclude them from causing harm there. However, it COULD deter them from doing it there, and maybe following their intended victim or victims to a less hardened place-grocery store, mall, laundromat, etc. It doesnt prevent the crime from happening; it just lowers the risk of it happening there.
Unfortunately, someone bent on destruction will likely pursue that goal to its end-either until they commit the act, or get caught preparing for it. We cannot apply rational thoughts & ideas to this, because they simply dont apply. It isnt rational to hate an institution to the point of killing its followers; normal thinking folks just dont do that. Visual security may, or may NOT, deter them; invisible security would be the same. Visible security at least has the command presence; and, if strategically placed, can cover one another (provided you have more than 2).
Layered security (visible out front, invisible within the church) is an option; however, remember one thing; if they are to remain invisible, they must do what the congregation does; that is kneel, pray, take communion, sing, etc (based on christian based religions). Otherwise, they arent invisible. This ties them up, and breaks concentration-they cannot be as vigilant when in a crowd, singing. Now, keeping them within the church area, up near the back, along the sides-even hidden from the congregation, but with a good view...that would work-but again, hidden means blending in-in an "active" crowd, that is kind of hard to do. You cannot scan with your head bowed, nor hear well when siging.