The hymn sung as the Blessed Sacrament is placed on the altar.
The last two verses of a longer hymn Saint Thomas Aquinas wrote for Corpus Christi, now sung at the exposition of the Blessed Sacrament.
When: At the beginning of Benediction, as the monstrance is placed on the altar.
Why: It names what is on the altar: the victim of the Cross, the same one, still opening heaven. Everything else at Benediction follows from that claim.