In light of recent suggestions about “union with Christ” mediating the New Perspective controversies, I thought this selection was interesting from Calvin’s Institutes:
5. But as Osiander has introduced a kind of monstrosity termed essential righteousness, by which, although he designed not to abolish free righteousness, he involves it in darkness, and by that darkness deprives pious minds of a serious sense of divine grace  ; before I pass to other matters, it may be proper to refute this delirious dream. And, first, the whole speculation is mere empty curiosity. He indeed, heaps together many passages of scripture showing that Christ is one with us, and we likewise one with him, a point which needs no proof; but he entangles himself by not attending to the bond of this unity. The explanation of all difficulties is easy to us, who hold that we are united to Christ by the secret agency of his Spirit, but he had formed some idea akin to that of the Manichees, desiring to transfuse the divine essence into men.  Hence his other notion, that Adam was formed in the image of God, because even before the fall Christ was destined to be the model of human nature. But as I study brevity, I will confine myself to the matter in hand. He says, that we are one with Christ. This we admit, but still we deny that the essence of Christ is confounded with ours. Then we say that he absurdly endeavors to support his delusions by means of this principle: that Christ is our righteousness, because he is the eternal God, the fountain of righteousness, the very righteousness of God. My readers will pardon me for now only touching on matters which method requires me to defer to another place. But although he pretends that, by the term essential righteousness, he merely means to oppose the sentiment that we are reputed righteous on account of Christ, he however clearly shows, that not contented with that righteousness, which was procured for us by the obedience and sacrificial death of Christ, he maintains that we are substantially righteous in God by an infused essence as well as quality. For this is the reason why he so vehemently contends that not only Christ but the Father and the Spirit dwell in us. The fact I admit to be true, but still I maintain it is wrested by him. He ought to have attended to the mode of dwelling–viz. that the Father and the Spirit are in Christ; and as in him the fulness of the Godhead dwells, so in him we possess God entire. Hence, whatever he says separately concerning the Father and the Spirit, has no other tendency than to lead away the simple from Christ. Then he introduces a substantial mixture, by which God, transfusing himself into us, makes us as it were a part of himself. Our being made one with Christ by the agency of the Spirit, he being the head and we the members, he regards as almost nothing unless his essence is mingled with us. But, as I have said, in the case of the Father and the Spirit, he more clearly betrays his views–namely, that we are not justified by the mere grace of the Mediator, and that righteousness is not simply or entirely offered to us in his person, but that we are made partakers of divine righteousness when God is essentially united to us.
6. Had he only said, that Christ by justifying us becomes ours by an essential union, and that he is our head not only in so far as he is man, but that as the essence of the divine nature is diffused into us, he might indulge his dreams with less harm, and, perhaps, it were less necessary to contest the matter with him; but since this principle is like a cuttle-fish, which, by the ejection of dark and inky blood, conceals its many tails,  if we would not knowingly and willingly allow ourselves to be robbed of that righteousness which alone gives us full assurance of our salvation, we must strenuously resist.