In his writings, the theologian and mysticist William of St-Thierry (early 12th century) elaborates a pneumatological conception centering around espousal dynamism. From this place, the author’s main formulations are reflected in the work Expositio super Cantica canticorum: First, William of St-Thierry understood love as a primacy of the Spirit; second, he elucidated the progress of beauty as the source of every good, referring to lovers’ reciprocity; and finally, he speaks of the union of the two lovers which brings them into only one Spirit. From these axes, we intend to understand and analyze Unitas spiritus, as an economic event in the life of a believer, from which our author articulates that the Holy Spirit makes possible the unity of Father God and Son, as well as that of God and mankind.