St. Cyprian of Carthage is one such source. He lived in the early third century, and is famous for opposing the Novatians and for his heroic martyrdom. He also wrote the seminal work De Unitate Ecclesiae (On the Unity of the Church) which describes in beautiful detail the nature of the Catholic Church, and why it is important to Christians.
To St. Cyprian, the existence of the Church is for one simple, yet elegant purpose. The Church exists to be salt for the earth, following the commandment of Christ. (Matt 5:13) Today we would say the Church exists to worship God, to uphold proper doctrine, even to save souls. All of these are very important, but they are means to an end. We want man to be saved because when he is saved, he is better than he could possibly be without it. According to Cyprian, the only source of this good is Jesus Christ:
This [schism] happens, beloved brethren, so long as we do not return to the source of truth, as we do not seek the head nor keep the teaching of the heavenly Master.
This truth is also proclaimed by St. Ambrose, who proclaims in the Hexameron:
The Church shines not of its own light, but with that of Christ and takes its own splendor from the Sun of righteousness, so that it can say ‘It is no longer I who live, but Christ lives in me’.
While the Church upholds this truth, Cyprian also states that the Church is meant to proclaim that truth. He compares the Church to the sun when he says:
Thus also the Church, shone over with the light of the Lord, sheds forth her rays over the whole world, yet it is one light which is everywhere diffused, nor is the unity of the body separated.This speaks to the fundamentally evangelical nature of the Church. What good is the light of the sun, if it does not enlighten everything? If it tried to do something else, it would cease being the sun. Indeed, eventually our sun will die when it loses its light. When the Church (and her members) do not proclaim Christ as part of their nature, they lose their purpose.
These truths have profound implications for our Church today. I see this denied in (at least) two ways. The first is what is a Catholicism of the believer which exists only within Church walls. This is what secular culture wants us to be like. Worship God all we want on Sundays, but make him another part of the pantheon of the secular gods of money, success, tolerance, choice, etc. When we are doing this, we are trying to suppress the light of Christ which is meant to shine through us. More often than not, we do this out of fear of mockery/rejection by the world. We don't wish to enlighten the world, we wish to co-exist with it.
Another way this is denied is through a "let it burn" mentality. In this scenario, the world is going to hell, there's absolutely nothing we can do to stop it, so we just hold onto the truth, forget the world, and let it burn into oblivion. I believe this temptation is very strong for my traditionalist brethren, especially in a culture so radically opposed to Christ. Sometimes we might even feel that way about the Church as a whole. Let the modernists ruin everything, as long as we keep our Latin Mass. The traditionalist movement doesn't exist to enrich the whole church, but rather to enrich our own desires. In this case we've erected barriers to the light shining forth. We reject "the mystery of the moon" in that we do not reflect Christ to the world, but rather our disdain for them.
I believe the writings of the Fathers will help to cure us of these two problems. They dealt with a culture just like ours, and in many cases worse. Pagan Rome permitted all kinds of abominations, and actively persecuted the small group known as Christians. They didn't need to fear rejection, rejection was certain. They needed to fear the sword coming down on their necks for professing Christ. Further, pagan society was worse than ours, but they never believed in "let it burn", but rather viewed their blood as the seed which would transform entire civilizations. These are the themes we will be covering in future installments of Cyprian's great treatise.