And Marvin had already prepared this bid for trust.
It was the 'treaty' sent by Napoleon himself.
Marvin intended to have Holmes present the treaty to the upper echelons of the Doomsday Party, thereby not only demonstrating Holmes's 'loyalty' and value to them but also informing them of the ambitions of Napoleon and his cohorts.
By the time Napoleon started to inquire about Alchemy Technology, the Doomsday Party would already be forewarned, and even if Napoleon eventually obtained the information, it would surely come at a great cost.
In contrast, the Church of Truth could take advantage of this opportunity to send Holmes to infiltrate the core of the Doomsday Party and gather intelligence on the Outer Gods.
The Outer Gods have always been shrouded in mystery, and up until now, most of what Marvin knew about them came from hearsay. If there were interactions between the Doomsday Party and the Outer Gods, wouldn't that provide valuable intelligence?