Two works of the pioneering “Dutch Radical” Allard Pierson are now available in English:
The Sermon on the Mount and Other Synoptic Fragments, 1878
Contents:
— On Certain Flaws in the Method of Criticism of the Synoptic Gospels (Parts I and II)
— Synoptic fragments
—- I. The Sermon on the Mount.
—- II. On fasting
—- III. In the grainfield
Original Dutch file: De Bergrede en andere synoptische fragmenten (I used this text for the translation)
See also the Google books file: De bergrede en andere synoptische fragmenten
Also by Allard Pierson, with Samuel Adrian Naber:
Probabilities [= Verisimilia], 1886
Original Dutch file: Verisimilia at archive.org
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Schweitzer, Albert. 2001. The Quest of the Historical Jesus. Fortress Press, on Pierson and Naber, and Verisimilia:
The radical Dutch thinkers denied the authenticity of the Pauline epistles. They could not accept that the thoughts contained in the epistles to the Romans, Corinthians and Galatians were the product of the middle of the first century and that they reflected the views of primitive Christianity. However, most of them still believed in the existence of Jesus, even though they maintained that it was the disciples who first made him into the Messiah. Only S. Hoekstra, Allard Pierson and Samuel Adrian Naber doubted whether the Gospels contained any historical information. (356)
In the Verisimilia Pierson and Naber offer the mythical interpretation; however, they hint at it more than developing it and giving reasons for it. (528)