What evidence is there that the men from the bar believe in and are afraid of witchcraft?
I believe you are referring to Chapter 10 when Ultima and Antonio go to cure Antonio's uncle from the curse the Trementina sisters have put on him. Three of the men in the bar are shocked that Ultima would dare to face Tenorio with only Antonio by her side. "They had not expected her to come into this place of evil." Even though the fourth man, Tenorio, doesn't turn around to look at them, Antonio can see his shoulders shaking with fear. Tenorio makes the sign of the cross that is supposed to work against a witch (bruja), but it doesn't affect Ultima. The men decide she must have powers that the devil himself has given her. The three men with Tenorio are so nervous and afraid of Ultima that Tenorio can't depend on them to act as witnesses against Ultima. All they can do is stand there and stare at Ultima. When Ultima tells Tenorio she knows when and where the curse was put on Lucas, the three men can't get out of the bar fast enough. They are more afraid of Ultima's powers than they are of Tenorio's anger for not supporting him.
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I think what you are looking for and referring to is from Chapter 10 when Uncle Tony, after having witnessed a meeting of three witches, daughters of Tenorio Trementina, who owns a bar in El Puerto was laid on a curse for seeing them in public. The town priest could not with his best of his ability lift the curse of poor innocent Uncle Tony, and so Ultima promises to cure Tony, bringing only Antonio by her side and to assist her and this was the first time that Tony witnessed Antonio's casted magic and spells.
Later, Tenorio and some of his side-kicks arrive at the Marez home and their main purpose was to make Ultima pay and get their revenge as one of Tenorio's daughters had died and he insisted that Ultima was the main culprit of her death and had to bear full responsibility and cost. Ultima, on the other hand wanted his daughters to lift the curse or suffer the disastrous consequences. She insisted that the daughters collect Lucas's hair for the curse after they came to Tenorio for a haircut but Tenorio vehemently denied the accusations. Tenorio was very fearful and scared about Ultima, thinking that she was a witch (bruja), so he placed holy needles that resembles a shape of a cross, that can ward off witches but Ultima manages to pass through it successfully, convincing them that she was not a witch but a simple human being.