Passage Summary: A scholar claimed a famous painter was in a fringe cult to explain his weird art, but there's no proof he was in it, and we know he actually belonged to a normal church.

Conclusion: It is improbable that the artist Hieronymus Bosch was a member of the religious group known as the Brethren of the Free Spirit.

Reasoning: There is documented evidence of Bosch belonging to a mainstream church, and there is a complete lack of evidence supporting his membership in the Brethren.

Analysis: The statement in question serves as a premise supporting the author's skeptical stance. By pointing out the 'absence of evidence' for the opposing theory, the author strengthens their own conclusion that the theory is likely incorrect. In the structure of the argument, this works alongside the 'positive evidence' (his mainstream church membership) to double-down on the rejection of Fraenger's hypothesis.