Neither Luther nor Zwingli devoted much attention to the doctrine of the Trinity. Both accepted the orthodox formulations of the oneness and threeness of God developed by the early councils, but neither felt compelled to elaborate on this teaching. At the beginning of his career Calvin too followed this pattern. The first edition of the Institutes contained only a meager statement on the Trinity; the word itself (sacra trinitas) is mentioned only twice. On the basis of these sparse statements, Pierre Caroli accused Calvin of Arianism. Calvin had no trouble disproving the false charge, but, from that time on, he became an adamant defender of the doctrine of the Trinity. This stance was reinforced by his close encounters with genuine anti-Trinitarians such as Servetus, Gentile, and Gribaldi. Gribaldi was a Paduan lawyer who freely disseminated his doubts about the Trinity among the Italian-speaking refugees in Geneva. At the instigation of Calvin, Gribaldi was banished from the city in 1557 for “sapping and perverting the chief article of our faith.”73 Four years earlier Servetus, for the same offense, had met a fate worse than banishment.
Calvin proved himself impeccably orthodox in his own formulations of the Trinity: “When we profess to believe in one God, under the name of God is understood a single, simple essence, in which we comprehend three persons, or hypostases” (Inst. 1.13.20). We must ask whether Calvin in adopting this classical definition of God did not violate his own principle of doing theology within the limits of revelation alone. Calvin was very sensitive to this question and sought to meet it head on. He was well aware that words such as ousia, hypostases, persona, and even Trinitas were non-scriptural terms. He once said, “I could wish they were buried, if only among all men this faith were agreed on: that Father and Son and Spirit are one God, yet the Son is not the Father, not the Spirit the Son, but that they are differentiated by a peculiar quality” (Inst. 1.13.5). Yet precisely because certain heretics, such as Arius, have used scriptural language to affirm nonbiblical concepts of God, it was necessary for Calvin to refute their errors by using words such as Trinity and Persons in order to render “the truth plain and clear” (Inst. 1.13.3).
Even in conceding this point, however, Calvin warned against a speculative incursion into the mystery of God’s essence. “Let us then willingly leave to God the knowledge of himself.” It was mere presumption for believers to “seek out God anywhere else than in his Sacred Word, or to think anything about him that is not prompted by his Word, or to speak anything that is not taken from his Word” (Inst. 1.13.21). Thus Calvin refused to twist the Scriptures in order to bolster the doctrine of the Trinity. Well-worn proof texts for the Trinity, such as the plural form of God (Elohim) in Genesis 1 or the thrice-repeated adulation of the seraphim in Isaiah 6:3 or Jesus’ statement, “I and my Father are one” (John 10:30) Calvin regarded as weak and spurious proofs for such an important doctrine.
Those who denied the Trinity surely struck a sensitive nerve of Calvin. He referred to them as “slippery snakes,” “babblers,” “rascals,” “certain frenzied persons such as Servetus and his like,” who traffic in “chicaneries” and “vile absurdities.” Why was the Trinity such an important issue for him? As we have seen, he was not interested in the metaphysical niceties of abstract theology, nor was he slavishly attached to traditional terminology. The Trinity was crucial because it was a witness to the deity of Jesus Christ and thus to the certainty of salvation procured by Him. The purpose of Calvin’s Trinitarianism was, like that of Athanasius, soteriological. He wanted to safeguard the biblical message, “God is manifest in the flesh,” against false interpretations, such as that of Servetus who “confounded the Son and the Holy Spirit with the creatures” (Inst. 1.13.22). Thus from the first edition of the Institutes onward, he placed the confession of the Trinity in a liturgical context, namely in the invocation of the Triune God at baptism. Baptism in the name of the Father, the Son, and the Holy Spirit witnessed both the unity and the triunity of God. The distinctions within the Godhead were seen in the particularizing characteristics of each “Person”: The free mercy of the Father by the sacrifice of His death, the Holy Spirit cleansing and regenerating, making us partakers of the benefits of the Son. Yet, lest anyone think that Christians worship three gods, the very oneness of baptism pointed to the essential unity of the three divine persons.