The Preaching Labors of the Apostles
In a short time the apostles of Christ by their preaching, which was inspired in them by the Holy Spirit, won multitudes of pagans to Christ, simple and unsophisticated people, as well as scholars and even kings. The apostles suffered much difficulty in their holy work enduring much grief and need. All the apostles, except John the Theologian, met a martyr’s end. The Apostle John the Theologian died in exile at a great old age. The number of Christians grew from year to year, even after the deaths of the apostles, and the Christian faith spread to all the ends of the earth.
The apostles preached mostly verbally; but in order that the teaching of Christ be better preserved, several of them under the inspiration of the Holy Spirit wrote it down in books. Therefore, these books are called the Holy Scriptures, as were the books of the prophets before Christ.
Two of the twelve apostles, Matthew and John, and two of the seventy apostles, Mark and Luke, each wrote a book about the life of Christ the Saviour on earth and His teaching. These four books are called the Gospels ("good news") because they preserve the teachings of Christ which the Saviour Himself called the Gospel. The apostles who wrote them were called the Gospel writers or Evangelists.
The Evangelist Luke wrote another book, the Acts of the Apostles or the deeds and labours of the apostles. This book records how the apostles spread the Christian faith in the first years.
Others of the apostles: James, son of Alphaeus, Peter, as well as John the Theologian and Jude (Judas), the brother of James, wrote seven general epistles, letters to Christians all over the world, and taught in these letters how to believe and to live according to the teachings of Christ. One of the letters is by the Apostle James, two — by Peter, three — by John, and one — by Jude.
The Apostle Paul wrote fourteen epistles to distant churches and people.
The Apostle John the Theologian, in addition to the Gospel and three epistles, wrote still another book, the Apocalypse or Revelation. In this book, there is found the future destiny of the Christian Church and of the whole world.
Of course, the apostles did not write in the books everything they taught and had heard from the Saviour. The Evangelist John the Theologian himself wrote, "But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written" (John 21:25).
The teachings and rules which the apostles conveyed to the Christians by their words and example firmly preserved Christianity. These teachings, which are given to them orally and only later written down by the saints, are called Apostolic or Holy Tradition.
Together with the apostles and after them, apostolic men or disciples of the apostles preached the Gospel. These pastors, teachers and fathers of the Church of Christ, spread and strengthened the Church of Christ by their speaking, writing, and by their holy lives.
(from: The Law of God by Fr. Seraphim Slobodskoy)