The average Christian of today more or less takes it for granted that all that he/she believes in and all that is part of the Church today were there from the beginning of Christianity. But history shows that many of the core Christian ideas that we take for granted today, have an evolutionary history of their own. And The Early Church Fathers had a major role in that.
Understanding the ideas and times of the Early Fathers can give us a better view of why Christian theology is the way it is now. And without these great men, the history of Christianity and the World would have been very different.
Some of the important ones are:
Tertullian, (ca. 155–230)
He introduced the term Trinity the way it is accepted by the Church today. He defined the concept of Trinity with the term Consubstantiality (Tres Personae, Una Substantia - Three Persons, One Substance). Unlike the modern Christian belief, he held that all people (Christians & non-Christians) will be in harmony with God ultimately (the greek term is Apokatastasis).
St. Irenaeus (2nd Century)
A disciple of Polycarp, one of his major influences was his idealogical battle against Gnosticism.
Clement of Alexandria (ca. 150 - ca.216)
With his emphasis on the importance of both knowledge and faith, he had a great impact on later Fathers like Origen, St. Jerome, etc. Like Tertullian, he believed in the Apokatastasis principle.
Origen (185–ca. 254)
A mystic and philosopher. His writings were perhaps the first intellectual attempt to describe and defend Christian principles. He mixed Platonism and Stoic philosphy with Christian principles, to give faith an intellectual basis. He too held the Apokatastasis principle of salvation for all irrespective of their religion.
Athanasius of Alexandria (293 – 373)
A contemporary of Arius. His defense of the official dogma against that of Arius and his followers, was the beginning of a long conflict with Arianism. Famous for the Athanasian Creed that rejected Arianism, Nestorianism, Monophysitism, and Macedonianism.
Basil of Caesarea (ca. 329 - 379)
Famous for his idealogical battles against Arianism and other heresies. He is well known for his charitable works as well.
Saint Ambrose, Bishop of Milan (ca.337 – 397)
Played the decisive role in suppressing Arianism and pushing Roman paganism to the background, which would have had a phenomenal impact on the future of Christianity.
St. Jerome (ca. 342 - 419)
Translation of the entire Bible into Latin (Vulgate). The Vulgate was to have a major impact on the faith, art & culture of Christian Europe for more than 1000 years. Some scholars even hold that it had an important role in the development of many European languages.
Saint Augustine, Bishop of Hippo (354 – 430)
An intellectual and philosophical giant both for Christianity and Western thought. And perhaps the most important person for Christianity after Jesus and Paul. Many of the popular Christian concepts today were either propounded or clarified by him. He expounded the concept of Original Sin as accepted by all major Churches today. Another important principle that came from him was the theory of Just War (fighting for a just cause), as opposed to the traditional "Turn the Other Cheek" principle. He also held that the Bible should be interpreted symbolically and not literally.
Gregory the Great (c. 540 – 604)
He had a major role in transforming the Papacy to a seat of real power.