Pierre Lambert de la Motte was born in Normandy in 1624 into a pious noble family. After studying law, and a brief but distinguished career in civil administration, he entered the Congregation of the Assumption, a lay association. In 1657, he volunteered as a missionary in Asia, was appointed Titular Bishop of Beirut and left for Asia in 1660. Arrived in Siam, he made pastoral visits in Cochinchina (Southern Vietnam) and throughout Vietnam, educating and ordaining native priests, giving courage to dispersed communities, trying to settle the many rivalries between missionary orders and to improve tense relations with the authorities, while living the deep spiritual life of a mystic of the Cross. He died in 1679. His work is part of the considerable development of missions in the seventeenth century under the leadership of Pope Gregory XV. In France, Pierre Lambert de la Motte played a key role in the birth of the Paris Foreign Missions Society in 1658, for China and South-East Asia. They were to work tirelessly, following the Roman Instructions of 1659: refusal of secularization and politicization, respect for indigenous cultures, accession of the local Churches to autonomy and consolidation of their link with the Holy See. The author describes the work accomplished by the Paris Foreign Missions Society in this part of Asia, shows the difficulties in developing them, as well as the extraordinary tenacity of an inspired man.