The lives of saints were filled with it.

The cathedral sculpture embodied it in every part.

The storied windows made it all the more impressive.

The missal painters wrought it not only into prayer books, but, despite the fact that hardly a trace of the belief appears in the Psalms, they illustrated it in the great illuminated psalters from which the noblest part of the service was sung before the high altar.

The service books showed every form of agonizing petition for delivery from this dire influence, and every form of exorcism for thwarting it.
All the great theologians of the Church entered into this belief and aided to develop it.