"Personal holiness"—what thoughts does this phrase engender?
If I had to guess, I would say your mind went to thinking about spiritual disciplines or moral performance. Or at least that's what mine did. The word "personal" has a way of making our brains forget God's self-attested holiness and focus only on our own, which we typically equate to nothing more than our conceived progress in fulfilling certain do's and don'ts.
Moral code can easily become our attempt to live up to God's holiness. Ontology gives way to function. Being is replaced by doing (and not doing). Welcome to planet frustration. This is the world of many followers of Christ—the disappointing drudgery of the religious treadmill.
But there is good news. The gospel doesn’t stop where sanctification begins. Our holiness is not isolated from God’s holiness—it is dependent upon it.
John Webster explains in his book Holiness that sanctification is not an "acquired sufficiency" and that a Christian's holiness must always be in reference to the "triune work of grace." He writes,
The Christian’s sanctity is in Christ, in the Spirit, not in se [in itself]; it is always and only an alien sanctity. Sanctification does not signal birth of self-sufficiency, rather it indicates a 'perpetual and inherent lack of self-sufficiency'.
Sanctification 'in' the Spirit is not the Spirit's immanence in the saint. Quite the opposite: it is a matter of the externality of sanctitas christiana [Christian holiness], the saint being and acting in another.
'Sanctification in the Spirit' means: it is not I who live, but Christ who lives in me. And 'Christ lives in me' means: by the Spirit's power I am separated from my self-caused self destruction, and given a new holy self, enclosed by, and wholly referred to, the new Adam in whom I am and in whom I act (84).