"And ye shall be holy unto me: for I the LORD am holy, and have severed you from other people, that ye should be mine. Leviticus 20:26"
In the Old Testament, the Israelites were separated unto God; the Almighty God made the Israelites His children and considered the other nations to be Gentiles.
The Israelites were considered holy and the Gentiles unholy or unclean. In the New Testament, Christians are called the children of God, that is, those who have made Jesus Christ their Savior and Lord.
These Christians are separated unto God as a royal priesthood, a holy nation, and a special people (1 Peter 2:9).
Our Father wants us to be conscious of who we are in Him and to live a holy life for Him so that He can boast of us. We don't have to intermingle with the world; if we love with the world, the love of the Father is not in us (1 John 2:15).
We have been called out of the world to shine as the lights to the world, so let us be proud to say we are holy instead of saying we are sinners saved by grace. We are no longer sinners but righteous.
Hallelujah!