Strive for peace with everyone, and for the holiness without which no one will see the Lord. – Hebrews 12:14
What is holiness?
The imputed holiness of Christ.
By his active obedience he perfectly fulfilled the commands of the law, and by his passive obedience, his voluntary sufferings, he fully satisfied the penalties and curses of the law. This holiness of Christ is ours by imputation! It is reckoned unto believing sinners. Certainly, without this mediatory holiness we cannot stand before God.
Inherent, internal holiness.
This is God’s supernatural infusing of holy principals, divine qualities, or supernatural graces into the soul by the Holy Spirit. This is the new nature that God created in righteousness and true holiness. These gracious habits of holiness are the candles of the Lord, set up in us to walk by. A holy heart is always attended with a holy life, and without this, there is no hope or possibility of ever seeing the Lord. The more these spiritual habits are acted and exercised, the more they are increased and strengthened. Holy habits are golden talents that must be employed and improved!
We have been separated from common use and set apart for the worship and service of God! God will have no communion with any that are not separated from the sinful practices and unholy courses of the world. He will own none, delight in none, acknowledge none, and receive none for his sons and daughters but such as are separated from all evil vices and unholy courses. God will be only theirs that are really his, and he will be altogether theirs that are wholly his! God will resign himself up to them who resign themselves up to him; he will give up himself to them that have given up their names and their hearts to him. He that does not dedicate himself really to God, wholly to God, only to God, and always to God on earth shall never come to a vision of God in heaven (2 Cor. 6:17-18)!
Thomas Brooks, Works, IV:37-47