Holiness is the nature of God.
What separates a believer form others is Holiness,

Of old God called out the Israelite from among the heathen to be a holy nation to himself.

In the new testament God called us Christians to be a holy nation to him.

1 Peter 2:9 But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light:

The opposite of holiness is unholiness, it is a character, lifestyle and mindset set in things like fornication, adultery,murder, hatred, variance, love of pleasure more than God, despisers of those that are good, witchcraft/ spiritism, heretics( unbiblical, or twisted bible teachings), idolaters, emulation, strife, disobedience to parents, Covenant breakers, and all such like.
Galatians 5:21

These are unholiness, which characterizes the people of the world.
Holiness means being separate from all the above totally hating and rejecting it but instead embrace peace, gentleness, love, humility, righteousness,patience, long-suffering, Meekness, temperance etc

Unholy people are like dogs, dogs cannot inherit the kingdom of God, they remain outside in the lake of fire.

Revelation 22:15 For without are dogs, and sorcerers, and whoremongers, and murderers, and idolaters, and whosoever loveth and maketh a lie.

Revelation 21:8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.

Thank God, he has saved us freely from all of these but we should continue to abide in him, him his love.

John 15:9 As the Father hath loved me, so have I loved you: continue ye in my love.

Jude 1:21 Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life.

Amen.

Keep yourselves in the Loving arms of the Lord Jesus, don't be carried away with diverse doctrines both secular or religious only hold to what is of Jesus.
Jesus is saviour.