The purpose of this study is to investigate the life and works of Sébastien Castellion, a French humanist and reformer of the 16th century. The reason why I am concerned about Castellion is that his idea of tolerance is very suggestive to us who live in religious plural society. In order to fulfill this goal I first introduce a dramatic but tragic life of Castellion briefly, and then examine his central idea through his works. In particular, I will focus my interest on the tolerance issue of the 16th century that broke out between Calvin and Castellion concerning the death of Michael Servetus in 1553. In this paper, I look into Castellion’s several works dealing with tolerance problem: I select five primary works to reveal Castellion’s tolerance thought: “Preface to the Latin Bible, Dedicated to Edward VI,” “Concerning Heretics,” “Reply to Calvin’s Book,” “Counsel to France,” and “Concerning Doubt and Belief.” Castellion was a prophet of his time, a rationalist before Rationalism, and an advocate of religious tolerance in an age of intolerance. He was certainly ahead of his time. I hope this study will provide a momentum for sincere and academic concern about Castellion and his tolerance thought.