The handwritten letter was written in 1776 and outlines Saint Junipero Serra’s plan to establish a chain of missions across California.
"Imagine what California is like in 1776. This idea that Saint Junipero Serra had was to establish missions where postmen or couriers could make the trip and stop that," said Scott Trepel from Siegel Auctions.
"It was kind of like he was establishing post offices along that route. But they were missions and they served an important purpose at that time," said Trepel.
His efforts in establishing these missions played a key role in the Spanish colonization of California.
The Saint is depicted in a statue in Ventura – and has become a controversial figure in recent years. Native Americans say the Spanish occupiers abused their ancestors and forced them to convert to Catholicism.
The letters are expected to sell for $100,000 or more at auction on Thursday.
