[margin: Silvester, under “superstition,” question 10; Silvester again, with Navarrus, Manual, chapter 11; Cajetan and Valencia on Secunda Secundae, question 96]
Sometimes it will be mortal, sometimes venial. It is mortal, first, if it is done from an express pact with a demon. Second, if someone knows that at least a tacit pact is involved, or has been warned of it, and does not wish to desist.
It is venial so long as someone is ignorant that there is such a pact with a demon, provided he is neither personally bound to know this nor has been warned. Understand this of the person who has not consulted experts. For although someone may not be bound by reason of office or state to know the matter, nevertheless, when he begins to doubt, he is bound to seek advice from learned people. If he does not do so, he is negligent. He could have known, and therefore he is not invincibly ignorant.
There is no sin when someone does it only in jest, without prejudice or danger to himself or another. Likewise there is no sin if someone labors under invincible ignorance and uses means that are either good or indifferent.