Rossini is often considered to be, above all, a secular composer, and, to be honest, some of his holy masses are, well, not exactly "holy"; the most obvious example are the two masses I posted several months ago: "Messa di Gloria" and "Messa di Milano" which, if it were not for the text and some genuinely spiritual music, could pass off as cantatas or even operatic material. This motion is contrasted, however, by a number of pieces that, if not exactly on par with more well-established sacred music, are surprisingly (for a composer who has "Il barbiere" and "L'italiana" in his resume) beautiful invocations of the holy sphere, including the present simple but breathtaking prayer for mixed chorus, composed around 1857.
"O salutaris Hostia" or "Oh, saving Host" is a section of one of the Eucharistic hymns written by St. Thomas Aquinas for the Feast of Corpus Christi. He wrote it for the Hour of Lauds in the Divine Office. It is actually the last two stanzas of the hymn "Verbum supernum prodiens". Rossini, however, only sets the first verse a verse rendering of which is provided below:
O saving Victim, open wide
The gate of Heaven to man below;
Our foes press on from every side;
Thine aid supply; Thy strength bestow.