In this published doctoral dissertation Mark Garcia takes his readers to the centre, or to one of the centres, of Calvin's theology. He argues that Calvin's idea of duplex gratia is to be understood in terms of union with Christ. Union with Christ simultaneously confers two distinct but yet inseparable gifts. One of them, justification, is sola fide, the counting righteous of the believer by the imputation of Christ's righteousness; the other, sanctification, is also through the instrumentality of faith but involves gracious renewal, the imparting of non-meritorious virtue by Christ's Spirit. Calvin's ‘mysticism’ has currently attracted some attention, but Garcia wisely steers around this as a possible source for Calvin's idea of union with Christ. Instead he pursues the theme by detailed treatments of three important Calvinian contexts. First his exposition in the later Romans commentary and the parallel insertions into the Institutes; then controversy with the Lutherans Westphal and Heshusius over the supper; and finally Calvin's polemical remarks about the rather off-centre Lutheran Andreas Osiander regarding the nature of Christ's righteousness, as these are found in the 1559 Institutes. Garcia thinks that the importance of Osiander for Calvin lies in his conviction that Osiander's position regarding Christ's righteousness indwelling the believer is where a consistent Lutheranism will inevitably lead. The author displays wide knowledge of his sources, and of the provenance and literary history of his main texts. New light is cast on the course of Reformation theology, and on Calvin's fellow Reformers such as Vermigli, with whom Calvin corresponded over different senses of ‘union’. Garcia deals with all these matters through clear, careful exposition which is by and large convincing. However, this reviewer is not fully persuaded that the union with Christ-duplex gratia formula has an illuminating parallel in Calvin's sacramentalism so as to underline or ‘vindicate’ (p. 193) its centrality. Union with Christ is certainly central to Calvin's understanding of both baptism and the lord's supper. Further, Garcia clearly shows that in each case Calvin thinks in terms of two matters which are distinct but inseparable – justification and sanctification in the case of the duplex gratia, the sign and what it signifies for the worthy recipient in the case of the sacraments. But this common or parallel structure in thought does not necessarily reveal a common view about the connection of justification and sanctification with union, any more than would reference to the distinctness but inseparability of the persons of the Trinity. Rather, it shows a mind which carefully discriminates about different issues in parallel ways, but not someone who uses these parallel structures to reinforce the case for the unio Christi duplex gratia doctrine, as Garcia calls it. In any case, that doctrine does not need such strengthening, since (with the author's help) once one is alerted to it, one begins to see duplex gratia all over the place in Calvin's corpus.