Keep your heart with all vigilance, for from it flow the springs of life. (Proverbs 4.23)

There is a war within the heart of every Christian. It is subtle and the heavy artillery often arrives as whispers.

The ground? What will we love?
The world and its offerings are tangible and immediate. And, yet, every time we foster that romance there is something in us that feels sick. The indulgence was sweet, but desperately unhealthy. And it comes with a crash.

This is sin. James tells us plainly that friendship with the world is hatred towards God (James 4:4). In our souls, we know it and we feel the degrading effects. This unhealthy feeling is a blessing. Cursed is the heart that isn't restless in the siege. But what do we do to address it?

Generally our first inclination is to go on a diet - to stop eating the world's sacchariny smorgasbord. This is good, but it is never enough. We need to develop a hunger and a love for good food.

Why do you spend your money for that which is not bread, and your labor for that which does not satisfy? Listen diligently to me, and eat what is good, and delight yourselves in rich food. Incline your ear, and come to me; hear, that your soul may live; and I will make with you an everlasting covenant (Isaiah 55.2–3)

This was the point of Thomas Chalmer's (1780-1847) famous sermon "The Expulsive Power of a New Affection."

An excerpt:

In a word, if the way to disengage the heart from the positive love of one great and ascendant object, is to fasten it in positive love to another, then it is not by exposing the worthlessness of the former, but by addressing to the mental eye the worth and excellence of the latter, that all old things are to be done away and all things are to become new. To obliterate all our present affections by simply expunging them, and so as to leave the seat of them unoccupied, would be to destroy the old character, and to substitute no new character in its place. But when they take their departure upon the ingress of other visitors; when they resign their sway to the power and the predominance of new affections; when, abandoning the heart to solitude, they merely give place to a successor who turns it into as busy a residence of desire and interest and expectation as before - there is nothing in all this to thwart or to overbear any of the laws of our sentient nature - and we see how, in fullest accordance with the mechanism of the heart, a great moral revolution may be made to take place upon it.

The Love of God and the Love of the World are Irreconcilable

This, we trust, will explain the operation of that charm which accompanies the effectual preaching of the gospel. The love of God and the love of the world, are two affections, not merely in a state of rivalship, but in a state of enmity - and that so irreconcilable, that they cannot dwell together in the same bosom. We have already affirmed how impossible it were for the heart, by any innate elasticity of its own, to cast the world away from it; and thus reduce itself to a wilderness. The heart is not so constituted; and the only way to dispossess it of an old affection, is by the expulsive power of a new one. Nothing can exceed the magnitude of the required change in a man's character - when bidden as he is in the New Testament, to love not the world; no, nor any of the things that are in the world for this so comprehends all that is dear to him in existence, as to be equivalent to a command of self-annihilation.
But the same revelation which dictates so mighty an obedience, places within our reach as mighty an instrument of obedience. It brings for admittance to the very door of our heart, an affection which once seated upon its throne, will either subordinate every previous inmate, or bid it away. Beside the world, it places before the eye of the mind Him who made the world and with this peculiarity, which is all its own - that in the Gospel do we so behold God, as that we may love God. It is there, and there only, where God stands revealed as an object of confidence to sinners and where our desire after Him is not chilled into apathy, by that barrier of human guilt which intercepts every approach that is not made to Him through the appointed Mediator. It is the bringing in of this better hope, whereby we draw nigh unto God - and to live without hope, is to live without God; and if the heart be without God, then world will then have all the ascendancy. It is God apprehended by the believer as God in Christ, who alone can dispost it from this ascendancy. It is when He stands dismantled of the terrors which belong to Him as an offended lawgiver and when we are enabled by faith, which is His own gift, to see His glory in the face of Jesus Christ, and to hear His beseeching voice, as it protests good will to men, and entreats the return of all who will to a full pardon and a gracious acceptance. It is then, that a love paramount to the love of the world, and at length expulsive of it, first arises in the regenerated bosom. It is when released from the spirit of bondage with which love cannot dwell, and when admitted into the number of God's children through the faith that is in Christ Jesus, the spirit of adoption is poured upon us - it is then that the heart, brought under the mastery of one great and predominant affection, is delivered from the tyranny of its former desires, in the only way in which deliverance is possible.

Additional Resources
Chalmer's Entire Sermon
What should we seek in the Scripture?
Tomorrow we should feel