Oh man, I'm so glad to have found this. I sang this in high school, either my junior or senior year. Singing this in the rotunda was one of the _highlights_ of my high school choir career. Having the alto part to brush up again is nice. But it's estos, not esto. Estos les digo roughly translates to "these things I say to you." Esto and les have to agree in number.
i have to disagree with the last part, in Spanish saying “estos les” is grammatically wrong in that sentence. so esto should be right. Anyway this song is so pretty i love it