The protagonist of The Earth Will Shake, Sigismundo Celine, is described as a descendant of Sigismundo Malatesta. Pages 80 to 85 of the original Tarcher hardcover describe Malatesta and his Malatesta Tempio. The temple is described as more pagan than Christian and Malatesta himself is described in fantastic terms.
I wondered if Wilson made it all up, and in fact, he didn't: Here is the Wikipedia entry on Malatesta (named "Sigismondo" in the entry), and here is the entry in English on his temple. The article in the French version of Wikipedia confirms more of the juicy details in Wilson's novel.
In 1906, Edward Hutton published the historical novel Sigismondo Malatesta, mostly sympathetic to its hero. It was slightly revised and reprinted under the title The Mastiff of Rimini in 1926.
Hutton's novel and Charles Emile Yriarte's Un condottiere au XV Siècle (1882) were among the main sources of American poet Ezra Pound's Malatesta Cantos (Cantos 8-11), first published in 1923. These are an admiring howbeit fragmentary account of Malatesta's career as warrior, lover and patron.