This article examines the concept of «good faith» generally espoused byTomás Segovia (1927-2011) through all his literary work. The search for the origins of the idea has led us to his post-war essays. Here we analyse the concept as used by Segovia in the post-war years, from the perspective of existentialism, very much in vogue at the time. We compare Segovia’s idea of «good faith» with «fidelity to truth and morality» as conceived by Sartre in the same period. We conclude that many of Segovia’s uses of the concept can be explained on the basis of Sartre’s ethic; but at the same time we observe that the Mexican Segovia expressed nuances of his own. In particular the writer achieved a genuine attitude of «good faith», above all in the field of language and literature, which in that context favours the revolutionary ethic propounded by Sartre. Thus, he reinterprets «good faith» as a dynamic, creative pattern of behaviour in literary work.