In his Institutes of the Christian Religion, John Calvin wrote with deep insight into the doctrine of the Trinity. He dismissed as "an absurd fiction" (I:XIII:29) the scholastic teaching on the eternal generation of the Son. To him, the idea that the Father eternally generated the Son's divine essence was the wost kind of theological speculation. Instead, he proposed that "the Godhead is absolutely of itself [autotheos]. And hence also we hold that the Son, regarded as God, without reference to his person, is also of himself [autotheos]; though we also say that, regarded as Son, he is of the Father. Thus his essence is without beginning, while his person has its beginning in God". (I:XIII:25). The Son, in his divine essence is I AM, the self-existing God. He does not derive his deity from the Father. He is Son because he has a Father, but he is God because he is God.
B. B. Warfield spelt out the value of Calvin's contribution to our understanding of the Trinity in a remarkable and influential essay. Much recent evangelical work on the Trinity owes a debt to Warefield's exposition of Calvin's teaching.
"In his assertion of the autotheos of the Son Calvin, then, was so far from supposing that he was enunciating a novelty that he was able to quote the Nicene Fathers themselves as asserting it "in so many words". And yet in his assertion of it he marks an epoch in the history of the doctrine of the Trinity. Not that men had not before believed in the self-existence of the Son as He is God: but that the current modes of stating the doctrine of the Trinity left a door for the entrance of defective modes of conceiving the deity of the Son, to close which there was needed some such sharp assertion of His absolute deity as was supplied by the assertion of His autotheos. If we will glance over the history of the efforts of the Church to work out for itself an acceptable statement of the great mystery of the Trinity, we shall perceive that is is dominated from the beginning to the end by a single motive - to do full justice to the absolute deity of Christ. And we shall perceive that among the multitudes of great thinkers who under the pressure of this motive have laboured upon the problem, and to whom the Church looks back with gratitude for great services, in better formulation of the doctrine or better commendation of it to the people, three names stand out in high relief, as marking epochs in the advance towards the end in view. These three names are those of Tertullian, Augustine and Calvin. It is into this narrow circle of elect spirits that Calvin enters by the contribution he made to the right understanding of the doctrine of the Trinity. That contribution is summed up in his clear, firm and unwavering assertion of the autotheos on the Son. By this assertion the homoousios of the Nicene Fathers at last came into its full right, and became in its fullest sense the hinge of the doctrine".
B. B. Warfield
From Calvin's Doctrine of the Trinity, in Calvin and Augustine. p. 283 & 284, (P&R, 1980).