I love the way Richard Foster defines holiness in Streams of Living Water:
“Holiness is not rules and regulations. Elaborate lists of dos and don’ts miss the point of a life hidden with God in Christ. No single standard of behavior is dictated by the word holy. All external legalisms fail to capture the heart of holy living and holy dying.
Holiness is sustained attention to the heart, the source of all action. It concerns itself with the core of the personality, the well-spring of behavior, the quintessence of the soul. It focuses upon the formation and transformation of this center.
Holiness is not otherworldliness. Its life is not found by developing logic-tight compartments of things sacred and things secular. We do not come into it by studiously avoiding contact with our manifestly evil and broken world.
Holiness is world-affirming. The holy life is found smack in the middle of every day life. We discount it while being freely and joyfully in the world without ever being of the world. Holiness sees the sacred in all things. It is integrative, synoptic, incarnational.
Holiness is not a consuming asceticism. It is not punishment for the sake of punishment. It neither despises nor depreciates the human body. And it never locates virtue or merit in ascetical exercises themselves.
Holiness is a bodily spirituality. It affirms the goodness of the human body and seeks to bring it into working harmony with the spirit. It utilizes appropriate Spiritual Disciplines for training the body and mind in right living. It is, in this sense, ascetical—but never for the sake of asceticism, always for the sake of training.
Holiness is not “works-righteousness.” We cannot muster up our own willpower to do good deeds and thereby become righteous. Sanctifying grace, just like justifying grace, is utterly and completely a work of...well, grace. It is unearned and unearnable. It is a god-initiated and God-sustained reality; we cannot do it, conjure it up, or make it happen.
Holiness is a “striving to enter in,” as Jesus tells us. Effort is not the opposite of grace; works is. Works has to do with merit or earning, and the effort we are called to undertake has nothing whatever to do with meriting or earning anything. In fact, the classical Disciplines—fasting and prayer, for example—have no virtue or merit whatsoever in and of themselves. They merely place us before God in such a way that he can begin building the kingdom-righteousness within us. (I say “merely” because I want to underscore that the virtue is all of God, but I certainly do not want to give the impression that our effort is nothing. In the economy of God it is a very important something. We will come to this presently.)
Holiness is not perfectionism. We do not by some act of divine fiat become sinless creatures incapable of any wrong action. As holy persons we can still make mistakes—and we do, with sorrowful regularity. We fail. We fall. Even so...
Holiness is progress in purity and sanctity. We are set apart for divine purposes. Holy habits deepen into fixed patterns of life. We experience a growing preponderance of right actions flowing from a right heart. We are ever in the process of becoming holy.
Holiness is not absorption into God. It does not mean the loss of our identity, our personhood. Through holy living we do not become less real, less whole, less human. Quite the opposite.
Holiness is loving unity with God. It is an ever-expanding openness to the divine Center. It is a growing, maturing, freely given conformity to the will and ways of God. Holiness gives us our truest, fullest humanity. In holiness we become the persons we were created to be.”