This entire selection is from Charles Hodge, Systematic Theology, vol 1 (Grand Rapids: Eerdmans; repr. Henrickson, 2001), 467.
“Calvin was accused by some of his contemporaries of teaching the incompatible doctrines of Sabellianism and Arianism. In a letter to his friend Simon Grynee, rector of the Academy of Basle, dated May, 1537, he says the ground on which the charge of Sabellianism rested, was his having said that Christ was ‘that Jehovah, who of Himself alone was always self-existent, which charge,’ he says, ‘I was quite ready to meet.’ His answer is: ‘If the distinction between the Father and the Word be attentively considered, we shall say that the one is from the other. If, however, the essential quality of the Word be considered, in so far as He is one God with the Father, whatever can be said concerning God may also be applied to Him the Second Person in the glorious Trinity. Now, what is the meaning of the name Jehovah? What did that answer imply which was spoken to Moses? I AM THAT I AM. Paul makes Christ the author of this saying’ (Calvin’s Letters.vol. i. pp. 55, 56, edit. Presbyterian Board, Philadelphia). This argument is conclusive. If Christ be Jehovah, and if the name Jehovah implies self-existence, then Christ is self-existent. In other words, self-existence and necessary existence, as well as omnipotence and all other divine attributes, belong to the divine essence common to all the persons of the Trinity, and therefore it is the Triune God who is self-existent, and not one person in distinction from the other persons. That is, self-existence is not to be predicated of the divine essence only, nor of the Father only, but of the Trinity, or of the Godhead as subsisting in three persons. And, therefore, as Calvin says, when the word God is used indefinitely it means the Triune God, and not the Father in distinction from the Son and Spirit.”