We use this hymn, called the Trisagion (thrice-holy) Hymn, often. It’s part of the introductory prayers that we use before almost every worship and prayer service; and we sing it emphatically just before the epistle reading in Divine Liturgy.
It comes most directly from Isaiah’s vision in Isaiah 6:3, where the seraphim sing: “Holy, holy, holy is the Lord of hosts, the whole earth is full of His glory.”
Dr. Bruce Beck, from Holy Cross School of Theology, has written about this text and associated it with the text of Psalm 41:3: “My soul thirsted for God, the Mighty One, the
Living One. When shall I come and appear before the face of God?” (his translation; see
St Vladimir’s Theological Quarterly 63:2 (2019) 161–189 Unbinding the Book: Toward a
Restoration of a Patristic Orthodox Hermeneutic of Scripture, Bruce N. Beck).
In this Psalm verse the words, “God”, “Mighty,” and “Living” are found together, and this is behind the angels singing the word, “Holy” three times with these attributes added.
When we say or sing this hymn, we are praising God in a full way, acknowledging his glory and energies, which sustain our life and all life. We are singing with the seraphim and all creation, witnessing to the One who loves us and His holiness, power, and immortality.