First in the more general sense of separation from all that is human and earthly. It thus denotes the absoluteness, majesty, and awfulness of the Creator in His distinction from the creature. In this use of the word, "holiness" is little more than an equivalent general term for "Godhead," and the adjective "holy" is almost synonymous with "Divine" (compare Dan 4:8,9,18; 5:11). Yahweh's "holy arm" (Isa 52:10; Ps 98:1) is His Divine arm, and His "holy name" (Lev 20:3, etc.) is His Divine name. When Hannah sings "There is none holy as Yahweh" (1 Sam 2:2), the rest of the verse suggests that she is referring, not to His ethical holiness, but simply to His supreme Divinity.
But, in the next place, holiness of character in the distinct ethical sense is ascribed to God. The injunction, "Be ye holy; for I am holy" (Lev 11:44; 19:2), plainly implies an ethical conception. Men cannot resemble God in His incommunicable attributes. They can reflect His likeness only along the lines of those moral qualities of righteousness and love in which true holiness consists. In the Psalmists and Prophets the Divine holiness becomes, above all, an ethical reality convicting men of sin (Isa 6:3,1) and demanding of those who would stand in His presence clean hands and a pure heart (Ps 24:3 f).
From the holiness of God is derived that ceremonial holiness of things which is characteristic of the Old Testament religion. Whatever is connected with the worship of the holy Yahweh is itself holy. Nothing is holy in itself, but anything becomes holy by its consecration to Him. A place where He manifests His presence is holy ground (Ex 3:5). The tabernacle or temple in which His glory is revealed is a holy building (Ex 28:29; 2 Ch 35:5); and all its sacrifices (Ex 29:33), ceremonial materials (30:25; Nu 5:17) and utensils (1 Ki 8:4) are also holy. The Sabbath is holy because it is the Sabbath of the Lord (Ex 20:8-11). "Holiness, in short, expresses a relation, which consists negatively in separation from common use, and positively in dedication to the service of Yahweh" (Skinner in HDB, II, 395).
A ceremonial holiness, corresponding to that of impersonal objects and depending upon their relation to the outward service of Yahweh. Priests and Levites are holy because they have been "hallowed" or "sanctified" by acts of consecration (Ex 29:1; Lev 8:12,30). The Nazirite is holy because he has separated himself unto the Lord (Nu 6:5). Above all, Israel, notwithstanding all its sins and shortcomings, is holy, as a nation separated from other nations for Divine purposes and uses (Ex 19:6, etc.; compare Lev 20:24).
But out of this merely ceremonial holiness there emerges a higher holiness that is spiritual and ethical. For unlike other creatures man was made in the image of God and capable of reflecting the Divine likeness. And as God reveals Himself as ethically holy, He calls man to a holiness resembling His own (Lev 19:2). In the so-called "Law of Holiness" (Lev 17 through 26), God's demand for moral holiness is made clear; and yet the moral contents of the Law are still intermingled with ceremonial elements (Lev 17:10 ff; 19:19; 21:1 ff). In psalm and prophecy, however, a purely ethical conception comes into view--the conception of a human holiness which rests upon righteousness and truth (Ps 15:1 f) and the possession of a contrite and humble spirit (Isa 57:15). This corresponds to the knowledge of a God who, being Himself ethically holy, esteems justice, mercy and lowly piety more highly than sacrifice (Hos 6:6; Mic 6:6-8).
The idea of holiness is expressed here chiefly by the word hagios and its derivatives, which correspond very closely to the words of the Q-D-SH group in Hebrew, and are employed to render them in the Septuagint. The distinctive feature of the New Testament idea of holiness is that the external aspect of it has almost entirely disappeared, and the ethical meaning has become supreme. The ceremonial idea still exists in contemporary Judaism, and is typically represented by the Pharisees (Mk 7:1-13; Lk 18:11 f). But Jesus proclaimed a new view of religion and morality according to which men are cleansed or defiled, not by anything outward, but by the thoughts of their hearts (Mt 15:17-20), and God is to be worshipped neither in Samaria nor Jerusalem, but wherever men seek Him in spirit and in truth (Jn 4:21-24).
In the New Testament the term "holy" is seldom applied to God, and except in quotations from the Old Testament (Lk 1:49; 1 Pet 1:15 f), only in the Johannine writings (Jn 17:11; Rev 4:8; 6:10). But it is constantly used of the Spirit of God (Mt 1:18; Acts 1:2; Rom 5:5, etc.), who now, in contrast with Old Testament usage, becomes specifically the Holy Spirit or Holy Ghost.
In several passages the term is applied to Christ (Mk 1:24; Acts 3:14; 4:30, etc.), as being the very type of ethical perfection (compare Heb 7:26).
In keeping with the fact that things are holy in a derivative sense through their relationship to God, the word is used of Jerusalem (Mt 4:5), the Old Testament covenant (Lk 1:72), the Scriptures (Rom 1:2), the Law (Rom 7:12), the Mount of Transfiguration (2 Pet 1:18), etc.
No doubt, as applied to believers, "saints" conveys in the first place the notion of a separation from the world and a consecration to God. Just as Israel under the old covenant was a chosen race, so the Christian church in succeeding to Israel's privileges becomes a holy nation (1 Pet 2:9), and the Christian individual, as one of the elect people, becomes a holy man or woman (Col 3:12). In Paul's usage all baptized persons are "saints," however far they may still be from the saintly character (compare 1 Cor 1:2,14 with 5:1 ff).
Orr, James, M.A., D.D. General Editor. "Definition for 'HOLINESS'". "International Standard Bible Encyclopedia". bible-history.com - ISBE; 1915.