Another letter (II Thess.) soon followed, giving more detailed advice. As the field of Paul's activity broadened, he was obliged more and more to depend upon letters, since he could not in person visit the churches which he had planted. Questions of doctrine as well as of practice which perplexed the different churches were treated in these epistles. To certain of his assistants, like Timothy, he wrote dealing with their personal problems. Frankly, forcibly, and feelingly Paul poured out in these letters the wealth of his personal and soul life. They reveal his faith in the making as well as his mature teachings. Since he was dealing with definite conditions in the communities to which he wrote, his letters are also invaluable contemporary records of the growth and history of the early Christian church. Thus between 30 and 60 A.D., during the period of his greatest activity, certainly ten, and probably thirteen, of our twenty-seven New Testament books came from the burning heart of the apostle to the Gentiles. [Sidenote: _Growth of the other epistles_] Similar needs impelled other apostles and early Christian teachers to write on the same themes with the same immediate purpose as did Paul. The result is a series of epistles, associated with the names of James, Peter, John, and Jude. In some, like Third John, the personal element is predominant; in others, the didactic, as, for example, the Epistle of James. [Sidenote: _Purpose of the Epistle to the Hebrews_] A somewhat different type of literature is represented by the Epistle to the Hebrews. Its form is that of a letter, and it was without doubt originally addressed to a local church or churches by a writer whose name has ever since been a fertile source of conjecture. The only fact definitely established is that Paul did not write it. It is essentially a combination of argument, doctrine, and exhortation. The aim is apologetic as well as practical. Most of Paul's letters were written as the thoughts, which he wished to communicate to those to whom he wrote, came to his mind; but in the Epistle to the Hebrews the author evidently follows a carefully elaborated plan. The argument is cumulative. The thesis is that Christ, superior to all earlier teachers of his race, is the perfect Mediator of Salvation. [Sidenote: _Value of the Epistles_] Thus the Epistles, originally personal notes of encouragement and warning, growing sometimes into more elaborate treatises, were made the means whereby the early Christian teachers imparted their doctrines to constantly widening groups of readers. At best they were regarded simply as inferior substitutes for the personal presence and spoken words of their authors. Like the Old Testament books, their authority lies in the fact that they faithfully reflect, in part at least, the greater revelation coming through the lives and minds of the early apostles. [Sidenote: _The larger group_]