Because it is written, Be ye holy; for I am holy - Leviticus 11:44. This command was addressed at first to the Israelites, but it is with equal propriety addressed to Christians, as the professed people of God. The foundation of the command is, that they professed to be his people, and that as his people they ought to be like their God. Compare Micah 4:5. It is a great truth, that people everywhere will imitate the God whom they worship. They will form their character in accordance with his. They will regard what he does as right. They will attempt to rise no higher in virtue than the God whom they adore, and they will practice freely what he is supposed to do or approve. Hence, by knowing what are the characteristics of the gods which are worshipped by any people, we may form a correct estimate of the character of the people themselves; and, hence, as the God who is the object of the Christian's worship is perfectly holy, the character of His worshipers should also be holy. And hence, also, we may see that the tendency of true religion is to make people pure. As the worship of the impure gods of the pagan moulds the character of the worshippers into their image, so the worship of Yahweh moulds the character of His professed friends into His image, and they become like him.

(9) Because it is written, Be ye holy; for I am holy.
(9) He shows that sanctification does necessarily follow adoption.

Because it is written,.... In Leviticus 11:44.
be ye holy, for I am holy: an argument the apostle knew must have weight with these persons, who were chiefly Jews, scattered abroad among the Gentiles, and had a value for the Scriptures of truth; and therefore, as the argument for holiness of life, from the nature and perfections of God, is strong, it must receive additional strength from this being the declared will of God, even their sanctification on this account; and though holiness, equal to God, is never to be attained to by a creature, yet so far as it is capable of it, it is desirable, because agreeable both to the nature and will of God, by all such who are truly his children, who love his name, adore his perfections, give thanks at the remembrance of his holiness, fear his goodness, and obey his will.

Scripture is the true source of all authority in questions of doctrine and practice.
Be ye . . . for I am--It is I with whom ye have to do. Ye are mine. Therefore abstain from Gentile pollutions. We are too prone to have respect unto men [CALVIN]. As I am the fountain of holiness, being holy in My essence, be ye therefore zealous to be partakers of holiness, that ye may be as I also am [DIDYMUS]. God is essentially holy: the creature is holy in so far as it is sanctified by God. God, in giving the command, is willing to give also the power to obey it, namely, through the sanctifying of the Spirit (1-Peter 1:2).

Leviticus 11:44.