Saint John Cassian
He codified the wisdom of the Desert Fathers.
Saint John Cassian was a pioneering ascetic and monastic who codified the wisdom of the Desert Fathers of Egypt in the early 5th century. A precursor to Saints Benedict and Dominic, he outlined basic rules for monks, like declaiming any property, all the way down to books and shoes. He laid the theological foundation for the seven deadly sins, and offered strategies to overcome them.
More controversially, he came into theological conflict with Augustine — and, a millennia later, with Protestant Reformers — for his belief that divine will animates but does not fully override human will. In other words, to paraphrase hugely, in mankind's creation, it inherited a bit of God's freedom, for good and bad. Uncharitable interpretations of this effectively de-platformed Cassian for centuries.
That's partly why the English translation of The Conferences, his collection of 24 dialogues with the Desert Fathers, was only completed in 1997. Clearly, the book had an impact on its translator, a Dominican Father from New York named Boniface Ramsey, more recently known as the whistleblower against the first Catholic cardinal to be defrocked over sexual abuse.