Creyente means 'believer'. While it can refer to religious belief, in this song, it takes on a more ironic and poignant meaning.
The singer says, "Ay, por tu culpa me dicen del creyente' / Porque te creo que por él poquito sientes" (Oh, because of you they call me 'the believer' / Because I believe you feel little for him). Here, 'creyente' highlights the singer's unwavering, almost naive, belief that Lupita will eventually choose him, despite her being with someone else. It's a word that captures his hopeful, yet perhaps deluded, devotion.
Carín León spends this whole song asking a married woman for a bigger slice of her week. He lays it out in the first verse without flinching: "Porque entre semana es la mujer de otro" (because during the week she is another man's wife), so he only gets weekends, and he wants more. His pitch is almost comically practical: "Esta semana dame más de dos diítas" (this week give me more than two little days), "Martes y jueves de mañanita" (Tuesday and Thursday first thing). Then comes the cover story, "Dile a tu vato que en pilates tienes cita" (tell your guy you've got a Pilates class), which is the joke the whole song is built on, and he delivers it completely straight.
Underneath the scheming there is something less funny. "Ya estoy cansado, ay, ya estoy cansado" (I'm tired, oh, I'm tired), he sings, "De despertarme y que no estés a mi lado" (of waking up without you next to me). He never asks her to leave anyone. He waits, and he says exactly what happens if she ever does: "Ese mismo día yo en el rancho te hago boda" (that very same day I'll marry you at the ranch). When he shrugs the husband off with "Y si el güey se entera, yo no tengo lío" (and if the guy finds out, that's no trouble for me), you can hear that he means it.