While John Calvin rightly preferred the biblical idiom of union with Christ to the words “deification” or “beatific vision,” Calvin’s theology still has overlaps with earlier Christian theology (and the later Puritan emphasis on the beatific vision). To my mind, Calvin matches the Bible’s presentation of these matters by speaking of communion with Christ.
Here are three passages where Calvin speaks about the beatific vision (the vision of God, deification, etc.). It is key to note two things: first, Calvin never sees any sort of essential transformation between God and people. We participate qualitatively, not by essence. Second, Calvin sees the vision of God as the end of salvation, but he is reticent to say much more because it goes beyond our ability to grasp.
Calvin on the Resurrection (Inst. 3.25.10)
But since the prophecy that death shall be swallowed up in victory (Hosea 13:14), will then only be completed, let us always remember that the end of the resurrection is eternal happiness, of whose excellence scarcely the minutest part can be described by all that human tongues can say. For though we are truly told that the kingdom of God will be full of light, and gladness, and felicity, and glory, yet the things meant by these words remain most remote from sense, and as it were involved in enigma, until the day arrive on which he will manifest his glory to us face to face (1 Cor. 15:54). “Now” says John, “are we the sons of God; and it does not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is,” (1 John 3:2). Hence, as the prophets were unable to give a verbal description of that spiritual blessedness, they usually delineated it by corporeal objects.
On the other hand, because the fervor of desire must be kindled in us by some taste of its sweetness, let us specially dwell upon this thought, If God contains in himself as an inexhaustible fountain all fulness of blessing, those who aspire to the supreme good and perfect happiness must not long for any thing beyond him. This we are taught in several passages, “Fear not, Abraham; I am thy shield, and thy exceeding great reward,” (Gen. 15:1). With this accords David’s sentiment, “The Lord is the portion of mine inheritance, and of my cup: thou maintainest my lot. The lines are fallen unto me in pleasant places,” (Ps. 16:5, 6). Again, “I shall be satisfied when I awake with thy likeness,” (Ps. 17:15). Peter declares that the purpose for which believers are called is, that they may be “partakers of the divine nature,” (2 Pet. 1:4). How so? Because “he shall come to be glorified in his saints and to be admired in all them that believe,” (2 Thess. 1:10). If our Lord will share his glory, power, and righteousness, with the elect, nay, will give himself to be enjoyed by them; and what is better still, will, in a manner, become one with them, let us remember that every kind of happiness is herein included.
But when we have made great progress in thus meditating, let us understand that if the conceptions of our minds be contrasted with the sublimity of the mystery, we are still halting at the very entrance. The more necessary is it for us to cultivate sobriety in this matter, lest unmindful of our feeble capacity we presume to take too lofty a flight, and be overwhelmed by the brightness of the celestial glory. We feel how much we are stimulated by an excessive desire of knowing more than is given us to know, and hence frivolous and noxious questions are ever and anon springing forth: by frivolous, I mean questions from which no advantage can be extracted.
But there is a second class which is worse than frivolous; because those who indulge in them involve themselves in hurtful speculations. Hence I call them noxious. The doctrine of Scripture on the subject ought not to be made the ground of any controversy, and it is that as God, in the varied distribution of gifts to his saints in this world, gives them unequal degrees of light, so when he shall crown his gifts, their degrees of glory in heaven will also be unequal. When Paul says, “Ye are our glory and our joy,” (1 Thess. 2:20), his words do not apply indiscriminately to all; nor do those of our Savior to his apostles, “Ye also shall sit on twelve thrones judging the twelve tribes of Israel,” (Mt. 19:28). But Paul, who knew that as God enriches the saints with spiritual gifts in this world, he will in like manner adorn them with glory in heaven, hesitates not to say, that a special crown is laid up for him in proportion to his labors.
Our Savior, also, to commend the dignity of the office which he had conferred on the apostles, reminds them that the fruit of it is laid up in heaven. This, too, Daniel says, “They that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever,” (Dan. 12:3). Any one who attentively considers the Scriptures will see not only that they promise eternal life to believers, but a special reward to each. Hence the expression of Paul, “The Lord grant unto him that he may find mercy of the Lord in that day,” (2 Tim. 1:18; 4:14). This is confirmed by our Savior’s promise, that they “shall receive an hundredfold and shall inherit everlasting life,” (Mt. 19:29). In short, as Christ, by the manifold variety of his gifts, begins the glory of his body in this world, and gradually increases it, so he will complete it in heaven.
Calvin on 2 Peter 1:4:
For we must consider from whence it is that God raises us up to such a height of honor. We know how abject is the condition of our nature; that God, then, should make himself ours, so that all his things should in a manner become our things, the greatness of his grace cannot be sufficiently conceived by our minds. Therefore this consideration alone ought to be abundantly sufficient to make us to renounce the world and to carry us aloft to heaven. Let us then mark, that the end of the gospel is, to render us eventually conformable to God, and, if we may so speak, to deify us.
But the word nature is not here essence but quality. The Manicheans formerly dreamt that we are a part of God, and that, after having run the race of life we shall at length revert to our original. There are also at this day fanatics who imagine that we thus pass over into the nature of God, so that his swallows up our nature. Thus they explain what Paul says, that God will be all in all (1 Corinthians 15:28,) and in the same sense they take this passage. But such a delirium as this never entered the minds of the holy Apostles; they only intended to say that when divested of all the vices of the flesh, we shall be partakers of divine and blessed immortality and glory, so as to be as it were one with God as far as our capacities will allow.
Calvin on Romans 6:5:
Ingrafted, etc. There is great force in this word, and it clearly shows, that the Apostle does not exhort, but rather teach us what benefit we derive from Christ; for he requires nothing from us, which is to be done by our attention and diligence, but speaks of the grafting made by the hand of God. But there is no reason why you should seek to apply the metaphor or comparison in every particular; for between the grafting of trees, and this which is spiritual, a disparity will soon meet us: in the former the graft draws its aliment from the root, but retains its own nature in the fruit; but in the latter not only we derive the vigor and nourishment of life from Christ, but we also pass from our own to his nature. The Apostle, however, meant to express nothing else but the efficacy of the death of Christ, which manifests itself in putting to death our flesh, and also the efficacy of his resurrection, in renewing within us a spiritual nature.