When someone says “se me ha ido el santo al cielo” that means he forgot something, something he had to do or something he wanted to say. Other case where we can hear this expression and which, according to uses, close in the expression origin, is for example when someone is going to say something and suddenly can’t say anything. Seeing him like that, the other people ask him, What did you want to say?, to this the first will answer: “ay, no lo sé, se me ha ido el santo al cielo”, that means he forgot what he wanted to say.
According to the legend, this expression comes from a day when a predicator made an allusion to a Saint during a mass but he didn’t remenber the name of him and, in order to get out of this embarrassment, he said:”se me ha ido el santo al cielo (like if the saint didn’t stay in his mind until the end of its sermon, that’s why he said the saint had gone to the sky).