The middle lines ( Bella premunt hostilia ) acknowledge the "hostile battles" or spiritual struggles that believers face, asking God for the "oak-like" strength ( robur ) to persevere.
Beyond its liturgical roots in Gregorian chant, "O Salutaris Hostia" has inspired centuries of sacred music. Advent Day 17: O Salutaris Hostia - Jesuits.org O Salutaris Hostia - Catholic Latin Hymn
In 1264, Pope Urban IV instituted the Feast of Corpus Christi and commissioned St. Thomas Aquinas to compose the liturgy and hymns for this new celebration. "O Salutaris Hostia" actually comprises the final two stanzas of a much longer hymn, Verbum Supernum Prodiens , which Aquinas wrote for the Hour of Lauds (Morning Prayer) in the Divine Office. The middle lines ( Bella premunt hostilia )
The hymn has a companion piece, "Tantum Ergo Sacramentum," which is similarly the final two stanzas of another of Aquinas's works, Pange Lingua . Together, these texts provide the spiritual and musical framework for Benediction. Thomas Aquinas to compose the liturgy and hymns