Isaiah 6:3

And one cried unto another Divided into two choirs, they sung responsively one to the other; and said, Holy, holy, holy is the Lord of hosts “God's holiness,” says Lowth, “or the superlative purity of his nature, implies in it all the rest of his attributes, especially his justice and mercy, which are dispensed by the most exact rules of rectitude. The Christian Church has always thought the doctrine of the Trinity to be implied in this threefold repetition of holy: as it is also intimated in several other passages of the Old Testament, particularly in that form commanded to be used in blessing the people, Numbers 6:24; and Isaiah 48:16, of this book;” where see the notes. Thus Jerome observes the design of their hymn was “to show that there is a Trinity in the one Godhead; and to testify, that, not the Jewish temple, as formerly, (for that was to be forsaken of God,) but the whole earth was full of his glory:” namely, of the effects and demonstrations of his glorious holiness, as well as of his power, wisdom, and goodness.