9. Holy. Pious

There are two corresponding words in the Hebrew, and both occur in Psalm 89: ὅσιος corresponds with chasid, "thy holy One," ver. 19; and ἅγιος with qadôsh, "the holy One" in ver. 18, the singular of the word translated ‘saints' in verses 5 and 7. ὅσιος, in the neuter plural, is rendered ‘mercies' in "the sure mercies of David" in Acts 13:3434And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David. (Acts 13:34).
The difference of the two words has been well expressed thus: God is holy, knowing good and evil perfectly ― wills absolutely good and no evil: and we are separated, set apart from evil or common use to Him that is ἅγιος ὅσιος, on the contrary, is the exercise of gracious suitable affections in the relationships in which we stand to God, to parents, &c.; also God in mercy to us, and Christ in whom they are displayed. Hence however, as suitable affections towards God practically constitute holiness, it is used in this sense for holiness. Its only occurrences are Acts 2:27; 13:34, 3527Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption. (Acts 2:27)
34And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David. 35Wherefore he saith also in another psalm, Thou shalt not suffer thine Holy One to see corruption. (Acts 13:34‑35) Tim. 2:8; Tit, 1:8; Heb. 7:2626For such an high priest became us, who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens; (Hebrews 7:26); Rev. 15:4; 16:54Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest. (Revelation 15:4)
5And I heard the angel of the waters say, Thou art righteous, O Lord, which art, and wast, and shalt be, because thou hast judged thus. (Revelation 16:5). Thus it is, as is its Hebrew equivalent, employed of God and man. It is goodness and grace in God ― piety and recognition of Him on man's part, and is so used of Christ as the One in whom all gracious qualities are concentrated.
ἅγιος is of much more frequent use.