He was born in Geneva to two French Huguenot refugees. The family returned to France after the Edict of Saint-Germain in 1562, and settled at Crest in Dauphiné, where Arnaud Casaubon, Isaac's father, became minister of a Huguenot congregation. Until he was nineteen, Isaac had no education other than that given him by his father. Arnaud was away from home for long periods in the Calvinist camp, and the family regularly fled to the hills to hide from bands of armed Catholics who patrolled the country. It was in a cave in the mountains of Dauphiné, after the St Bartholomew's Day's Massacre, that Isaac received his first lesson in Greek, based on Isocrates' Ad Demonicum.
