For the essence of God being simple and undivided, and contained in himself entire, in full perfection, without partition or diminution, it is improper, nay, ridiculous, to call it his express image, (charakte). But because the Father, though distinguished by his own peculiar properties, has expressed himself wholly in the Son, he is said with perfect reason to have rendered his person (hypostasis) manifest in him. And this aptly accords with what is immediately added, viz.,that he is “the brightness of his glory.” The fair inference from the Apostle’s words is, that there is a proper subsistence (hypostasis) of the Father, which shines refulgent in the Son. From this, again it is easy to infer that there is a subsistence (hypostasis) of the Son which distinguishes him from the Father.
The same holds in the case of the Holy Spirit; for we will immediately prove both that he is God, and that he has a separate subsistence from the Father. This, moreover, is not a distinction of essence, which it were impious to multiply.
Calvin, John. The John Calvin Collection: 12 Classic Works . Waxkeep Publishing. Kindle Edition.