When travelling over long distances and mountainous terrain it can make a lot of difference to the pleasure of riding. Even if the purpose of your pilgrimage is penitential, there is no need to make carrying excess weight part of the penance. Never forget, with no apology for the repetition, that you have to carry water and emergency food, and water is heavy. There are a wide variety of racks to support your panniers. The mountings and securing nuts must be checked daily.
Panniers come in a variety of styles and sizes.
In any event prudence suggests that the pilgrim will contain his kit within the panniers in a dustbin liner or similar protection to ensure that rain and dust are excluded. An important consideration is the fixing of the pannier to the rack. If they simply hook on they will just as simply bounce off if you hit a pothole, so they should have a positive fixing that ensures that they are locked on. All the fixings should be regularly checked. A bar bag can be useful for carrying valuables, camera etc., because they are easily detachable to carry around with you.