What is the theological place of the Holy Spirit with respect to the forgiveness of sins in the sacrament of penance? This study examines the role of the Spirit in the theology of sacramental forgiveness of Thomas Aquinas (1224/5-1274), who is often blamed for the Geistvergessenheit of Western theology. In the first part of this study it is shown that in Thomas' theology notions like guilt and forgiveness function within the context of a relationship of friendship between God and human beings. Constitutive for this relationship is the indwelling of God, which is 'appropriated' to the Holy Spirit. It is explained that Thomas understands appropriation, i.e. the practice of ascribing to divine Persons individually what belongs to the divine essence in general, as a part of proper God-talk, which takes into account the limitations of our language vis-a-vis God. In the second part of this study, it is argued that the notion of the causality of the sacrament of penance, i.e. that it effects the forgiveness of sins that it signifies, can only be evaluated properly if the sacrament of penance is not only seen as prolongation of the incarnation, i.e. the visible mission of the Son, but also as accompanied by the continous invisible mission of the Holy Spirit. Eric Luijten (1964) has been a research-fellow of the Catholic Theological University at Utrecht, the Netherlands, and at present is rector of studies of the Arienskonvikt, the priest seminary of the archdiocese Utrecht and the diocese Groningen.