The Trisagion, the Thrice-Holy, is the short sequence of prayers with which nearly every Orthodox service and private rule begins. At its heart is the hymn “Holy God, Holy Mighty, Holy Immortal, have mercy on us,” taken up, the tradition holds, from the very song of the angels who cry “Holy, holy, holy” before the throne of God.
The Trisagion prayers gather several ancient prayers into a threshold: the hymn to the all-holy God, the “Glory to the Father,” the plea “All-holy Trinity, have mercy on us,” the Lord's Prayer, and the calling on God's mercy. To pray them is to place oneself, before asking anything, in the presence of the Holy One, and to begin every prayer where prayer belongs, in worship.
Because they open everything, the Trisagion prayers are among the first a new Orthodox Christian learns by heart. Whatever rule you keep, you will begin here; and to know them is to be able to begin to pray anywhere, at any time, with the Church's own first words.