One of the difficulties that Calvin had to face was that his insistence on the idea that the Son and the Holy Spirit were autotheos in the same way as the Father left others with the impression that he was denying the Son’s eternal generation (and, by implication, the Holy Spirit’s eternal procession, too). This misunderstanding was based on a fundamental inability to distinguish the two modes of discourse in God – the one personal and the other essential. Classical trinitarian orthodoxy had always said that the Son was eternally generated from the Father as a person but that, as a substance or being, He was fully God. It was not that distinction that caused the problem, but Calvin’s insistence that the Son’s personal generation had nothing to do with His possession of the divine essence. Most people had always assumed that, when the Father begat the Son, He communicated the divine essence to Him, just as happens in human birth. A child’s human nature is the same as that of his parents, but it is also derived from them, and this is how “eternal generation” was most naturally understood.
Calvin denied that. As he believed, the Son was fully and eternally in possession of the divine essence in His own right, or a se ipso (“from Himself”), as this was expressed in Latin. He argued that this “aseity” of the Son was not only biblical, but that it was also the only way that an essential subordination of the Son to the Father could be avoided. What Calvin was trying to say was that the language of generation and procession described personal relationship and not essential origin. In eternity, neither generation nor procession could have any temporal meaning, so that these words described a permanent state of affairs and not a process by which the Father somehow extended His being to the Son and the Holy Spirit. It was a logical deduction from existing orthodoxy, but Calvin’s clear separation of essence from personal relation was a development of earlier tradition that many of his contemporaries interpreted as a departure from it.
Calvin’s originality in this respect was strongly influenced by his battles against the anti-trinitarians of his time. In his view, for the traditionally orthodox to say that the Father alone was autotheos was to play into the hands of these heretics because they said exactly the same thing. Indeed, on their own terms, the heretics could be justified if such a view were to be adopted by the orthodox because, if the Son and the Holy Spirit were not autotheos, they could not be God at all. It would then matter very little whether the heretics were neo-Arians, neo-Sabellians (modalists), or whatever, because the fundamental point that held trinitarian orthodoxy together would have been conceded in advance.
From: God Has Spoken: A History of Christian Thought by Gerald Bray (Wheaton: Crossway, 2014), pp. 1,004-1,005.