"Of course we should obey God; of course we should love him with all our heart, mind, soul, and strength. The Bible tells us so. But if we ever give the impression that it is possible to do this on our own, not only do we make the gospel irrelevant, but we suggest that the law is in fact a lot weaker in its demands than it really is. Legalism demeans the law by reducing its standards to the level of our competence. There is a hopelessly misleading adage that one hears from time to time, and from people who ought to know better, that God does not require of us anything that we cannot give. This implies either that God requires less than perfection, or that perfection is less than perfect because we can achieve it. In fact the law of God was not framed according to the sinful human ability to keep it, but as an expression of the perfect character of God.
"To say what we should be or do and not link it with a clear exposition of what God has done about our failure to be or do perfectly as he wills is to reject the grace of God and to lead people to lust after self-help and self-improvement in a way that, to call a spade a spade, is godless."
In practical terms, if we as preachers lay down the marks of the spiritual Christian, or the mature church, or the godly parent, or the obedient child, or the caring pastor, or the responsible elder, or the wise church leader, and if we do this in a way that implies that conformity is simply a matter of understanding and being obedient, then we are being legalists and we risk undoing the very thing we want to build up. We may achieve the outward semblance of conformity to the biblical pattern, but we do it at the expense of the gospel of grace that alone can produce the reality of these desirable goals. To say what we should be or do and not link it with a clear exposition of what God has done about our failure to be or do perfectly as he wills is to reject the grace of God and to lead people to lust after self-help and self-improvement in a way that, to call a spade a spade, is godless."
Graeme Goldsworthy, Preaching the Whole Bible as Christian Scripture: The Application of Biblical Theology to Expository Preaching, 118-119.