In his epistle to Diognetus, the second century Christian Mathetes explained the relationship between Christians and the world by making an analogy relating Christians to the soul, and the rest of the world, to the body:
To sum up all in one word— what the soul is in the body, Christians are in the world. The soul is dispersed through all the members of the body, and Christians are scattered through all the cities of the world. The soul dwells in the body, yet is not of the body; and Christians dwell in the world, yet are not of the world. The invisible soul is guarded by the visible body, and Christians are known indeed to be in the world, but their godliness remains invisible. The flesh hates the soul, and wars against it, though itself suffering no injury, because it is prevented from enjoying pleasures; the world also hates the Christians, though in nowise injured, because they abjure pleasures. The soul loves the flesh that hates it, and [loves also] the members; Christians likewise love those that hate them. The soul is imprisoned in the body, yet preserves that very body; and Christians are confined in the world as in a prison, and yet they are the preservers of the world. The immortal soul dwells in a mortal tabernacle; and Christians dwell as sojourners in corruptible [bodies], looking for an incorruptible dwelling in the heavens. The soul, when but ill-provided with food and drink, becomes better; in like manner, the Christians, though subjected day by day to punishment, increase the more in number. God has assigned them this illustrious position, which it were unlawful for them to forsake.
In his understanding, Christians are to direct the world like the soul guides the body. The world will struggle against Christians like the body, with its desires, fights against the soul. And yet, just as the body is good, so the world itself, created by God, should also be seen as good. It is not the body, but its inordinate passions, its desires without direction, which is the problem. Likewise, people attached to world, when left unchecked, will blindly follow all kinds of impulses to unsatisfactory ends.
To be a Christian, to follow Christ, is not to abandon the world but to love it as the soul loves the body, recognizing the reactions of the world against them is like the reactions of their body against the dictates of their soul. The body is saved in and through the salvation of the soul, therefore the world finds its salvation through the Christians.
And so we often find our body is at war against us, that it is out of control with its wants and desires. It is not that its needs are worthless. They should be met, but the way the body engages those needs, through various pleasures and pains, it is easy to become subdued by its desires and to pursue needs in excess with the hope of greater pleasure in return. We can see this in the way gluttony works. It is good to eat, indeed, we need to eat in order to survive. When we get a good meal, we enjoy it; and so our body suggests, thanks to the pleasure, to eat more than necessary, to eat and continue to eat until we are over-stuffed. The more we do this, we will find our capacity for eating increases, and so the more we will end up having to eat in order to feel full. We will eat and eat, with some sense of pleasure, but since it is in excess, we will be harming ourselves in the process, opening ourselves up to a wide range of physical ailments. When we do not give in to the inexhaustible desire to eat, when we realize we have had enough to satisfy our needs and it is unhealthy to have more, our body with its desires often will revolt: the lack of satisfaction will turn into a kind of pain and suffering which will tempt us to give in to and eat more. Only through self-discipline, warring against the body, do we find ourselves capable of victory. And yet, it is for the body’s own good that we do this, even if the body and the sensations it offers does not know or comprehend what we do for it. We fight against the body for the good of the body because we recognize the body itself is worth saving by keeping it healthy. This is how we are to follow Paul when he tells us that the body must be kept under control by the Spirit. We must not read it as if he were saying that our flesh is evil, but rather with the realization that he is saying its impulses are easily disordered without the proper direction of the Spirit:
But I say, walk by the Spirit, and do not gratify the desires of the flesh. For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh; for these are opposed to each other, to prevent you from doing what you would (Gal. 5:16-17 RSV).
The Spirit and the flesh are often fighting against each other. The Spirit is the higher principle, and so it is capable of directing the flesh because it is filled with reason, while the flesh is not. The flesh will direct us and guide us according to its many functions and the desires it has to fulfill them, but it does not know how to do so, and so will lead us blindly through the passions to engage its many needs. The Spirit should counter the promptings of the flesh with reason, taking what is good in the body and directing it to follow its natural desires, using its higher intuition as a way to place reasonable limits on the body.
And so, following the understanding of Mathetes, Christians are to guide the world like the soul to the body. They are able to do this thanks to the grace they receive in and through their union with the Holy Spirit. This is what makes Christians, Christian, that they have been anointed by the Spirit of Christ, making them priest-kings in the world working for the salvation of the world itself (cf. Rev. 5:10). It is a function which they should serve with selfless devotion, not using it for their own worldly gain but for the sake of the rest of the world. If and when they use their role selfishly, they have abandoned what it means to be Christian and are to be seen like the rest of the world.