The following passages are taken from George Tavard’s book, The Starting Point of Calvin’s Theology. Tavard’s work is a significant and unique contribution to Calvin studies, as it introduces readers to Calvin’s practically unknown book, Psychopannychia, which examines the immortality of the soul. In addition to highlighting Calvin’s thorough knowledge of the Church Fathers and Scripture, Psychopannychia also reveals “Calvin’s rootedness in the medieval mystic tradition and his deep catholicity, even as he took steps that would define him as a Reformer.” In chapter 10, Tavard highlights the orthodoxy of Calvin’s Trinitarian theology.
The originality of his presentation of Trinitarian doctrine emerges from his understanding of the notion of “person” in God. This had been a point of debate in medieval speculation. The stream of thought that originated in the writings of Boethius and was chiefly represented by Thomas Aquinas understood personhood as “a distinct subsistence in a rational nature.” A person is that entity which is endowed with reason and subsists in itself. On the whole, reflection on the dogma of the Trinity has mostly followed this line of approach (p. 177).
Another stream of thought, however, that goes back to Richard of St. Victor in the twelfth century, and was chiefly emphasized by John Duns Scotus at the end of the thirteenth, understood personhood as, seen negatively, the incommunicability, or, positively, the uniqueness, of a spiritual or rational being. A person is that spirit which is itself and no other. Personhood belongs to the order of existence rather than of subsistence. In the God of the Christian revelation it designates a dimension of divinity that is so unique that it cannot be communicated and shared. That there are in God three such dimensions is at the core of the revelation of Christ. Abba, the Father of the Logos incarnate, is neither the Son nor the Spirit, and vice versa twice repeated. The Father is known to believers in a glass, darkly, through the further revelation of the filiation of the Second Person and the procession of the Third” (p. 177).
According to Tavard, the 1559 version of the Institutes appears to bring these two approaches together.
Starting with the Greek term hypostasis used in Hebrews 1:3, Calvin explains in the Latin versions: ‘There is no doubt that he [the Apostle] designates some subsistence in which he [the Father] differs from the Son.’ This is further clarified with the remark: “Person I call a subsistence in the essence of God, which, related to the others, is distinguished by an incommunicable property” (p. 178).
However, in the French version of 1561, Calvin adds a new aspect and attempts to explain the term “subsistence” in terms of indwelling. “This word [hypostasis] implies a subsistence residing in the essence of God, which, being related to the others, is distinct from them by virtue of an incommunicable property.” So interestingly, for his French readers, Calvin explicates subsistence by the term “residence”-a term that is “borrowed from the well-documented spiritual experience of sensing God ‘indwelling’ in the Christian soul” (p. 178).
Tavard goes on to say that due to Calvin’s pastoral concerns, he tended to focus his biblical commentaries in a moral direction and that this aspect of Calvin has been advanced by his predecessors more so than the mystical roots of his Trinitarian theology.
Nonetheless, the indwelling of the three Persons in the soul was the model he followed when he explained the Father, the Son, and the Spirit as three mutual indwellings in the divine ousia. Each Person is a specific indwelling, a residence, in this essence of God. ‘But as it [la Parole, the Word] can have been in God only as residing in the Father, this shows the subsistence of which we speak, which, though it is joined with the essence by an inseparable link, nonetheless has a special mark by which to be different from it.’ The divine Word subsists and dwells in God the Father. [...] It remains that the writing of Psychopannychia had turned his theological perspective in the direction of the soul’s interiority, exactly in that inner dimension of humanity-Augustine’s ‘intimiority’-in which Christian faith and experience have located the indwelling of the Three Persons (p. 179).
 Quoted from the back cover of the book. Inst. of 1559/61, I, ch. 13, n. 2.
 Personam voco subsistentiam in Dei essential quae, ad alias relata, proprietate incommunicabili distinguitur (I, ch. 13, n. 6).
 Inst. of 1559/61, I, ch. 13, n. 6.