Mathetes describes the relationship of Christians to the world as the soul to the body.
"To sum up all in one word— what the soul is in the body, that are Christians 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." (Mathetes 6)Notice that Mathetes is not talking about Christianity but about Christians. Christianity is not so much a religion as it is the universal membership of those who have found faith in God and in His Word, Jesus Christ. Christianity is not primarily about believing a certain set of doctrines, although all Christians share a common faith and understanding of God, nor is it primarily found in achieving orthodoxy with some religious group, although there is similarities in their form and mode of worship, but Christianity is primarily a relationship with God and with His Christ. Christianity is both individualistic and corporate. Here Mathetes is speaking of the relationship of the individuals to the world, not necessarily of the relationship with the corporate religious expressions to the world.
One of the chief aspects of Christianity that Mathetes highlights is its universality. In that day the idea of a worldwide religion was something new. Yes, in Mathetes day most people worshiped the Roman gods but they were still the Romans' gods. Previously, many had worshiped the Greek gods but they were still the Greeks' gods. Christianity, however, is a religion that is not "owned" by any nation or by some oligarchy, Christianity is the free association of individuals, first with God and then with each other. Christianity is unowned and uncontrolled, except by its God who has "first place in everything." (Collossians1:18) The universal appeal of Christianity is not found in joining a group but in coming to know someone; you are a Christian because of who you know not what group you join. Once coming to know God, people then freely join themselves with other knowers (or believer).
The second aspect that Mathetes highlights is the preserving and protecting power of Christians in the world.
"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." (Mathetes 6)There is a story of a king of Israel who found favor with God. King Manassah was an evil king and had reigned for fifty five years. As a result God pronounced destruction on Israel, but before God's punishment was executed another king arouse, a good king, named Josiah. Because of his goodness, God delayed His judgment while Josiah was alive. He promised Josiah, "Behold, I will gather you to your fathers and you shall be gathered to your grave in peace, so your eyes will not see all the evil which I will bring on this place and on its inhabitants." (2 Chronicles 34:28) As long as Josiah was alive, Israel would have peace. He was as a preserver and protector of Israel against the pending judgement of God.
The same is true for Christians. There are many reasons why God should judge our nations: violence, abortion, and debauchery, but there are also millions of reasons why He doesn't judge us; millions of believing Christians. It is similar to when Abraham pleaded with God for Sodom and Gomorrah, God said, "If I find in Sodom fifty righteous within the city, then I will spare the whole place on their account." (Genesis 18:26) Eventually Abraham negotiated God down from the original fifty people required to spare Sodom, "I will not destroy it on account of the ten." (Genesis 18:32)
But its not just in the protection from Judgement that Christians preserve and protect the world, its also in the releasing of blessing. It is as the time when Joseph was sold into slavery and became the servant of Potiphar. As Joseph served Potiphar, God blessed all that Potiphar had. "It came about that from the time he made him overseer in his house and over all that he owned, the Lord blessed the Egyptian's house on account of Joseph; thus the Lord's blessing was upon all that he owned, in the house and in the field." (Genesis 39:5) I believe that there is a blessing on my company simply because I am there. I believe that part of the reason my company has prospered is because God's blessing on my life spills over on everyone and everything around me. Christians not only act as a buffer between the world and God's Judgement but they also act as a conduit for His blessing. It is how we become salt and light in the world. This is the role that God has assigned to Christians in the world. As Mathetes said,
"God has assigned them this illustrious position, which it were unlawful for them to forsake." (Mathetes 6)Or as Jesus said, "You are the salt of the earth; but if the salt has become tasteless, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled under foot by men. You are the light of the world. A city set on a hill cannot be hidden; nor does anyone light a lamp and put it under a basket, but on the lampstand, and it gives light to all who are in the house. Let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven." (Matthew 5:13-16)