To live in the world is to be fully immersed in sensuality, to live for the sake of experience and pleasure and according to the whims of worldly opinions. To live in the world is to be moved by the eight worldly winds of gain and loss, status and disgrace, censure and praise, pleasure and pain, to be swept away by the passions of the world. To live in the world is to seek happiness and satisfaction in the conditioned, that which is impermanent, subject to change and dissolution.
The Buddha said that life is uncertain, that death may come at any moment, so we should practice heedfulness, the way to the Deathless, and freedom from the yokes of sensuality, becoming, views, and ignorance. In the Bible, too, one can find this kind of exhortation to live a life against the current of worldly passions and concerns for the sake of a greater happiness, to live heedfully and with wisdom:
What is the source of quarrels and conflicts among you? Is not the source your pleasures that wage war in your members? You lust and do not have; so you commit murder. You are envious and cannot obtain; so you fight and quarrel. You do not have because you do not ask. You ask and do not receive, because you ask with wrong motives, so that you may spend it on your pleasures. You adulteresses, do you not know that friendship with the world is hostility toward God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God. Or do you think that the Scripture speaks to no purpose: "He jealously desires the Spirit which He has made to dwell in us"? But He gives a greater grace. Therefore it says, "God is opposed to the proud, but gives grace to the humble." Submit therefore to God. Resist the devil and he will flee from you. Draw near to God and He will draw near to you. Cleanse your hands, you sinners; and purify your hearts, you double-minded. Be miserable and mourn and weep; let your laughter be turned into mourning and your joy to gloom. Humble yourselves in the presence of the Lord, and He will exalt you.
Do not speak against one another, brethren. He who speaks against a brother or judges his brother, speaks against the law and judges the law; but if you judge the law, you are not a doer of the law but a judge of it. There is only one Lawgiver and Judge, the One who is able to save and to destroy; but who are you who judge your neighbor?
Come now, you who say, "Today or tomorrow we will go to such and such a city, and spend a year there and engage in business and make a profit." Yet you do not know what your life will be like tomorrow. You are just a vapor that appears for a little while and then vanishes away. Instead, you ought to say, "If the Lord wills, we will live and also do this or that." But as it is, you boast in your arrogance; all such boasting is evil. Therefore, to one who knows the right thing to do and does not do it, to him it is sin. (James 4)
It is the craving for sensual desires that give rise to greed, hatred, and delusion, leading to quarrels and conflict. To live by the whims of these passions, one 'makes a friend of the world' and turns away from the noble life, from truth and peace and the Deathless. Our actions and intentions must be cleansed, purified. We must give up some of the short-term happiness and pleasure we experience from following our passions because they lead to long-term harm and suffering; and we must do what might initially be difficult for us to do because it will ultimately lead to our long-term welfare and happiness. We must be honest and practice right speech. And above all, we must not be heedless.
Although many have escaped into the forests, mountains, caves, and other remote places of the earth to help them renounce the world, which can be helpful, 'the world' itself goes with them. One can't escape the world simply by fleeing from civilization. Renunciation, then, is ultimately an internal turning away from our craving and desire once we begin to discern the drawback of sensual pleasures and the limitations of feeding our hunger for pleasant feelings and experiences.
Unfortunately, renunciation is often perceived as a negative word, especially in the West—a word that implies depriving oneself of something essential to living a full and happy life. But in the Buddha's dispensation, as well as in the teachings of Christianity, renunciation actually means the opposite of this—it's a word implying the relinquishment of something unessential to living a full and happy life.
One of the main reasons I find monasticism so appealing, I think, is that it's a way of life specially designed to help foster this kind of internal renunciation on the external level. Monastic life is like a spiritual training ground for those who either have trouble cultivating this spirit of renunciation while living a worldly life (like me) or who simply desire to devote themselves fully to a life of renunciation.