Concise Bible Dictionary:

Holiness has been described as “a nature that delights in purity, and which repels evil.” Adam and Eve were “innocent,” not holy; for though they might have delighted in purity, they did not repel the evil of Satan. God is ever holy; in heaven there is no evil to separate from, and He was holy, consistent with His perfection in everything, before there was any evil. The Spirit is the Holy Spirit though He is down here where sin is, and the Lord Jesus when in this sinful world was holy, harmless, and undefiled. God is called “the Holy One of Israel” (Isa. 30:15), and the Lord Jesus “the Holy One” (Mark 1:24; Acts 3:14).
The Israelites having been redeemed out of Egypt, and separated to God, it was said to them, “Thou art a holy people unto the Lord thy God: the Lord thy God hath chosen thee to be a special people unto himself, above all people that are upon the face of the earth” (Deut. 7:6). They were viewed as the chosen of God, as set apart for Him. This should have led to practical holiness, as God said, “Be ye holy, for I am the Lord your God” (Lev. 20:7). The Christian also is sanctified and justified, and Christ is made of God sanctification to him (1 Cor. 1:30), referring to the separative call of God, and the means and measure of his sanctification. As new created in Christ he partakes of the divine nature, so that holiness is followed. He is chastened also by the Father of spirits in order to his being partaker of God’s holiness.
One has said, “The Christian is called holy because he is set apart for God absolutely, according to the rights won by Christ in His death, and made good when he is born again, and thus set apart in a real way; and more perfectly, and with more intelligence, when he is sealed by the Holy Ghost, as cleansed by the blood of Christ.” Upon this are based the practical exhortations: “As he which hath called you is holy, so be ye holy in all manner of conversation” (2 Cor. 7:1; 1 Thess. 4:7; Heb. 12:14; 1 Pet. 1:15; 2 Pet. 3:11).

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