Albert Schweitzer's book "The Quest of the Historical Jesus" is considered by most scholars as the foundational work for understanding historical Jesus studies. Why? The book is not merely Schweitzer's personal view but a summary of EVERY significant scholarly book or essay on Jesus from Hermann Reimarus (1774) to the beginning of the Twentieth Century. Not until the final chapter does Schweitzer final adds his own view in 1906. The book as whole though is an overview of ALL scholarly study on the subject.
Prior to Reimarus, if someone wanted to understand Jesus the consensus was they needed to look no further than the Gospels of Matthew, Mark, Luke and John. Reimarus wrote the first complete model of a Jesus of History who was different than the Christ of Faith. For Reimarus, Jesus was a want-to-be Messiah figure who pushed to forcefully for Palestine liberation and ended up getting himself killed. The Disciples, according to Reimarus, had too much invested in the movement so they stole the body and circulated the myth of the risen Christ in order to perpetuate the movement.
Once Reimarus opened the door to the possibility that there was a historical Jesus who was not the same as the confessional Christ, there were a flood of additional theories along that line ranging from the truly bizarre (Karl Bahrdt, Jesus was a pawn of a secret society called the Essenes that used him as a tool to persuade the masses to give up hope in a political Messiah and focus instead on spiritual renewal, 1782) to the controversial (Bruno Bauer, there never was any person named Jesus. You’ll have to read the chapter on him for his explanation of how CHRISTianity could have started without a Christ, 1840).
The book conveys an incredible amount of research on the subject. The insight is brilliant but the sheer volume of content can be overwhelming so the discussion will be focused on Chapters 2, 3, 4, 6, 8, 11, and 18. Which cover the broad themes of Rationalistic explanations of Jesus (ch. 2, 3), Fictitious lives of Jesus (ch. 4), growing skepticism of rationalism (ch. 6), the rise of Jesus-myth explanations (ch. 8), the theory Jesus never even existed (ch. 11) and scholars’ position on the subject going into the 20th century (ch. 18).
This book was so influential that after over 125 years of constant writing on the subject, no major work was published for nearly 60 years after Schweitzer’s work because most believed he had closed the case of the subject.