It is often said that union is key to connecting justification and sanctification, the forensic and the renovative. In that light, Calvin’s discussion of the motivation for good works is surprising for the way that he counts union one among several other biblical grounds for sanctification.

[Philosophers], while they wish particularly to exhort us to virtue, announce merely that we should live in accordance with nature. But Scripture draws its exhortation from the true fountain. It not only enjoins us to refer our life to God, its author, to whom it is bound; but after it has taught that we have degenerated from the true origin and condition of our creation, it also adds that Christ, through whom we return into favor with God, has been set before us as an example, whose pattern we ought to express in our life. . . .

Then the Scripture finds occasion for exhortation in all the benefits of God that it lists for us, and in the individual parts of our salvation. Ever since God revealed himself faith to us, we must prove our ungratefulness to him if we did not in turn show ourselves his sons [Mal. 1:6; Eph 5:1; I John 3:1]. Ever since Christ cleansed us with the washing of his blood, and imparted this cleansing through baptism, it would be unfitting to befoul ourselves with new pollutions (Eph. 5:26; Heb. 10:10; I Cor. 6:11; I Peter 1:15, 19]. Ever since he engrafted us into his body, we must take especial care not to disfigure ourselves, who are his members, with any spot or blemish [Eph. 5:23-33; I Cor. 6:15; John 15: 3-6]. Ever since Christ himself, who is our Head, ascended into heaven, it behooves us, having laid aside love of earthly things, wholeheartedly to aspire heavenward (Col 3:1ff]. Ever since the Holy Spirit dedicates us as temples to God, we must take care that God’s glory shine through us, and must not commit anything to defile ourselves with the filthiness of sin [I Cor. 3:16; 6:19; II Cor. 6:16]. Ever since both our souls and bodies were destined for heavenly incorruption and an unfading crown [I Peter 5:4], we ought to strive manfully to keep them pure and uncorrupted until the Day of the Lord [I Thess. 5:23; cf. Phil 1:10]. These, I say, are the most auspicious foundations upon which to establish one’s life. One would look in vain for the like of these among the philosophers, who, in their commendation of virtue, never rise above the natural dignity of man. (Calvin, The Institutes, III.vi. 3)