(This excerpt is from J.I. Packer's book, REDISCOVERING HOLINESS)
What does it mean to behave naturally? What sort of behaviour, now, is natural to the child of God? THE CHRISTIAN'S NATURE: A widespread but misleading line of teaching tells us that Christians have two natures; an old one and a new one. They must obey the latter while denying the former. Sometimes this is illustrated in terms of feeding one of your two dogs while starving the other. The misleading thing here is not the reminder that we are called to holiness and not to sin, but that the idea of "nature" is not being used as it is used both in life and in Scripture. (see, for example, Romans 2:14, Ephesians 2:3). The point is that "nature" means the whole of what we are, and the whole of what we are is expressed in the various actions and reactions that make up our life. To envisage two "natures," two distinct sets of desires, neither of which masters me till I choose to let it, is unreal and bewildering because it leaves out so much of what actually goes on inside me.
The clearer and more correct thing to say, is this: we were born sinners by nature, dominated and driven from the start -- and most of the time unconsciously -- by self-seeking, self-serving, self-deifying motives and cravings. Being united to Christ in new birth through the regenerating work of the Spirit has so changed our nature that our heart's deepest desire (the dominant passion that rules and drives us now) is a copy, faint but real, of the desire that drove our Lord Jesus. That was the desire to know, trust, love, obey, serve, delight, honour, glorify and enjoy His heavenly Father -- a multi-faceted, many-layered desire for God, and for more of Him than has been enjoyed so far. The focus of this desire in Jesus was upon the Father, whereas in Christians, it is upon the Father and the Son together (and the latter especially). But the nature of the desire is the same. The natural way for Christians to live is to let this desire determine and control what they do, so that the fulfilling of the longing to seek, know and love the Lord becomes the mainspring of their life.
Augustus Toplady, one of the pioneers of England's evangelical revival in the eighteenth century, showed that he understood this when he wrote:
Object of my first desire, Jesus, crucified for me;
All to happiness aspire, only to be found in Thee.
Thee to praise, and Thee to know, constitute my bliss below;
Thee to see, and Thee to love, constitute my bliss above.
Whilst I feel Thy love to me, every object teems with joy;
May I ever walk with Thee, For 'tis bliss without alloy.
Let me but Thyself possess, total sum of happiness:
Perfect peace I then shall prove, heaven below and heaven above.
The momentous truth that thus emerges is that to "walk with Christ," as Toplady puts it, in the path of holy discipleship, is the life for which the hearts of Christians truly long. From this follows the equally momentous truth that obeying the promptings of indwelling sin (the sin that still marauds in the systems of Christians though it no longer masters their hearts) is not what they really want to do at all, for sinning is totally unnatural to them.
Why then do we ever do it -- let alone make a habit of doing it, as notoriously as we sometimes do? Partly, no doubt, because we fail to recognize sin for what it is, through ignorance of God's standards. Partly too because we yield to the nagging pull of temptation, giving way though we know that we should not and need not. But partly too because we let ourselves be deceived into supposing that to give way to this or that inordinate desire -- for food, drink, pleasure, ease, gain, advancement, or whatever -- is what we really want to do. Again and again it appears that Christians are not sufficiently in touch with themselves. They do not know themselves well enough to realize that, because of the way in which their nature has been changed, their hearts are now set against all known sin. So they hang on to unspiritual and morally murky behaviour patterns, and kid themselves that this adds to the joy of their lives. Encouraged by Satan, the grand master of delusion, they feel (feelings as such, of course, are mindless and blind) that to give up these things would be impossibly painful and impoverishing, so though they know they should, they do not. Instead they settle for being substandard Christians, imagining they will be happier that way. Then they wonder why their whole life seems to them to have become flat and empty. The truth is that they are behaving in a radically unnatural way, one that offers deep-level violence to their own changed nature. In doing what they think they like, they are actually doing what their renewed heart -- if they would only let it speak -- would tell them that it dislikes intensely, not only because it brings guilt and shame before God but, more fundamentally, because it is in itself repulsive to the regenerate mentality. The regenerate heart cannot love what it knows God hates. So these Christians are behaving unnaturally, occupying themselves in activities against which their own inner nature revolts. Such behaviour is always bad medicine, producing sadness, tension, and discontent, if not worse.
Backsliding: There is a venerable Christian term for this condition; backsliding, a recurring word-picture in Jeremiah 2:19, 3:22, 5:6, 14:7, 15:6. Particularly pertinent to our present discussion is the first of these passages, where God says through the prophet, "Your wickedness will punish you; your backsliding will rebuke you. Consider then and realize how evil and bitter it is for you when you forsake the Lord your God and have no awe of Me, declare the Lord, the Lord Almighty."
Three centuries ago the Puritan commentator, Matthew Henry, spelled out the implication of these words as follows: Observe here, (1) The nature of sin; it is forsaking the Lord as our God; it is the soul's alienation from Him, and aversion to Him. Cleaving to sin is leaving God. (2) The cause of sin; it is because His fear (awe) is not in us. Men forsake their duty to God, because they stand in no awe of Him, nor have any dread of His displeasure. (3) The malignity of sin; it is an evil thing and a bitter ... an evil that is the root and cause of all other evil ... It is not only the greatest contrariety to the divine nature but the greatest corruption of the human nature. (4) The fatal consequences of sin; as it is in itself evil and bitter, so it has a direct tendency to make us miserable ... it will certainly bring trouble upon thee ...and the justice of the punishment will be so plain, that thou shalt not have a word to say for thyself. (5) The use and application of all this; Know therefore (consider, then), and see it, and repent of thy sin, that so the iniquity ... may not be your ruin.
The unnatural act of backsliding, then, is always to be avoided, both because it provokes our holy heavenly Father to discipline and correct us in a punitive way (as is further explained in Hebrews 12:5-10), and also because, at some stage and in some measure, bitterness and misery are its ultimate and inescapable fruit. We must realize that all sin has the nature of suicidal, self-impoverishing madness, in the Christian life no less than elsewhere. To see this, and accordingly to commit oneself to follow one's heart by running in the path of God's calling and commands as hard and as fast as one can, is the directional basis of holiness. Since that is the most truly natural course for any Christian to follow, it holds out a hope of deep happiness, heart-happiness, here and now, that can never be attained otherwise.