The importance of Semler, sometimes called" the father of German rationalism," in ‘the history of theology and the human mind is that of a critic of biblical and ecclesiastical documents. He was not a philosophical thinker or theologian, though he insisted, with an energy and persistency before unknown, on certain distinctions of great importance when properly worked out and applied, e.g. the distinction between religion and theology, that between private personal beliefs and public historical creeds, and that between the local and temporal and the permanent elements of historical religion. In other words, Semler began the process of trying to separate the influence of personal belief and theology - from what a professor or teacher agreed to teach Publically. His great work was that of the critic, which means that he made great efforts to attack the Bible.

Though to some extent anticipated by the English "deist" Thomas Morgan, Semler was the first to take due note of and use for critical purposes the opposition between the Judaic and anti-Judaic parties of the early church. He led the way in the task of discovering the origin of the Gospels, the Epistles, the Acts of the Apostles, and the Apocalypse. He revived previous doubts as to the direct Pauline origin of the Epistle to the Hebrews, called in question Peter’s authorship of the first epistle, and referred the second epistle to the end of the 2nd century. In other words, Semler tried to establish a rivalry between the early Messianic Christians and the Gentile Christians, and then used those theories to suggest that the Bible had been tampered with, by one group or the other (depending on which Biblical book). He suggested that Peter never wrote 1 Peter, and suggested that Peter also Never wrote 2 Peter, and that it should be dated to more than 100 years after Peter. IN other words, according to Semler, 1 & 2 Peter are fraudulent in their claims and authenticity.

He wished to remove the Book of Revelation altogether from the Bible. In textual criticism Semler pursued further the principle of classifying MSS. in families, adopted by R. Simon and J. A. Bengel. In church history Semler did the work of a pioneer in many periods and in several departments. Friedrich Tholuck pronounces him "the father of the history of doctrines," and F. C. Baur "the first to deal with that history from the true critical standpoint." At the same time, it is admitted by all that he was nowhere more than a pioneer. (This means that he is admired by those who came after him, in his work of being among the first professors of Theology to attack the Authenticity of the Bible and Jesus Christ)

Tholuck gives 171 as the number of Semler’s works, of which only two reached a second edition, and none is now read for its own sakes.

When discussing the issue of a Textual Critic, Semler was among the first to attack the Bible openly. If the attack on the Bible had come from Catholic Theologians, then Protestants would have understood the implications and the problems. But many of the Protestant Theologians by the early 1800s had been greatly affected by philosophers who saw in religion the manifestation of all sorts of problems, both real and imagined. 

Theologians in German quickly gained control of most of the Universities, and then used these positions to lead astray as many of the students as possible. While everywhere else the Protestant Reformation had been celebrated for bringing in liberty and knowledge of the Bible, professors were using theology to undermine the Protestant Reformation as much as possible. The work that Semler and a few others began - were continued by their successors, many of whom came to the United States and taught in American Seminaries, while the majority of Americans were thinking that most of their future pastors were actually at Seminary learning about reasons to Trust the Bible, rather than reasons why God and the Bible should be understood as Fraudulent.

Sources: Ravenous Wolves: Textual Criticism and the Abandonement of the Reformation- A History of HIgher Criticism and German Theology by Lichtenberger. In addition, the works of the subjects of this biography, and Early Editions of Ency. Brit.