Theologian and mystic, and so called from the monastery of which he was abbot, b. at Liège about 1085; d. at Signy about 1148. William came of a noble family, and made his studies at the Benedictine Monastery of Saint Nicaise at Reims, together with his brother Simon. Here both embraced the religious life, and were raised to the abbatial dignity, Simon of St. Nicolas-aux-Bois, Diocese of Laon, and William at St-Thierry near Reims in 1119. Prior to this William had known St. Bernard, and had formed with him a close intimacy, which lasted for life. His greatest desire was to live with the saint at Clairvaux, but the latter disapproved of the plan and imposed on him the duty of remaining in charge of the souls which Providence had confided to him. However after having assisted (1140) at St-Médard near Soissons at the first general chapter of the Benedictines, where he suggested wise regulations, William, on the pleas of long infirmities and more and more attracted to a life of retirement, resigned his dignity as abbot (1135), and withdrew to the Cistercian abbey at Signy (diocese of Reims); he did not venture to retire to Clairvaux lest his friend Bernard would refuse to accept his abdication. Here, amid almost constant suffering, he divided his free time between prayer and study. According to a contemporary annalist his death occurred about the time of the council held at Reims under Pope Eugenius; this council took place in 1148, and his death should be placed in this year or the preceding. The necrology of his abbey dates it 8 September., in any case it was prior to that of St. Bernard (20 August, 1153).
Besides his letters to St. Bernard, William wrote several works which he himself enumerates, somewhat incorrectly, in one of his letters. Among them are: "On the solitary life" (De vita solitaria); "On the contemplation of God" (De Deo contemplando), modelled on the "Confessions" and "soliloquies" of St. Augustine; "The nature and dignity of Divine love" (De natura et dignitate amoris), the sequel to the preceding; "The Mirror of Faith" (Speculum fidei); "The Enigma of Faith" (Aenigma fidei); "On the Sacrament of the Altar" (De sacramento altaris liber), setting forth against the monk Rupert his views on the manner of the presence of Jesus Christ in the Eucharist; "Commentaries on the Canticle of Canticles" (complete), the first according to the conferences of St. Bernard, the second according to St. Ambrose, the third according to St. Gregory the Great; "Commentary on the Epistle of St. Paul to the Romans".
William was the first to deal with the errors of Abelard and to urge St. Bernard against him. He wrote "The Dispute against Abelard" (Disputatio adversus Petrum Abelardum), in which are arranged under twelve heads the errors which were condemned by the Council of Sens; the "Disputation of the Fathers against the dogma of Abelard" (Disputatio catholicorum Patrum adversus dogmata Petri Abelardi) was a reply to Abelard's apology; "On the errors of Guillaume de Conches" (De erroribus Guillelmi de Conchis) was a defence of the true idea of the Trinity. To these works should be added a life of St. Bernard, of which William wrote only the first chapters. His works were first printed by Tissier in "Bibliotheca Cisterciensis", IV (Bonofonte, 1669), and republished in P.L., CXXX (Paris, 1885).
BULAEUS, Historia universitatis Parisiensis (Paris, 1665), II, 763; Histoire litteraire de la France (Paris, 1869), XII; KUTTER, Wilhelm von St. Thierry ein Reprasentant der mittelalterlichen Frommigkeit (Gliessen, 1898).