Protection of the Sepulchre doesn't affect Knights

[Kristoffer O]

The protection is affected by MR, meaning that units with high MR will not benefit from the spell as often as units with low MR. It might still be useful to cast it as it is more important to protect the stronger troops from banishment.

[AreaOfEffect]

My suggestion would be to increase the effectiveness of your priests by equipping spell penetration items. You may not prefer to do this with all your priests, but I would surely do this for my profit.