Reginald Heber was an English bishop and hymn writer whose short life (he died at 42 in Calcutta) left an outsized mark on Christian liturgy. Holy, Holy, Holy was one of his finest works, written as a hymn for Trinity Sunday.
The three-fold repetition of 'holy' echoes the seraphim in Isaiah 6, the creatures around God's throne who cry out the holiness of God. Heber's genius was to make theology singable: he takes the abstraction of Trinity, the unreasonable claim that God is three and one, and gives it a body so clear that a child can sing it and a theologian can rest in it.
The hymn ends with a question that hangs in the air: only the redeemed can glimpse God's holiness and live. Heber knew his theology and his craft; every line earns its place.