Robert Letham, Union with Christ: In Scripture, History, and Theology (Phillipsburg, NJ: Presbyterian & Reformed, 2011), 123–28:

Ten Theses on Union with Christ and Transformation

1. The union we enjoy with Christ is more real and more fundamental than the union we have with members of our own bodies. . . .
2. This is not a union of essence—we do not cease to be human and become God or get merged into God like ingredients in an ontological soup. This is not apotheōsis. . . .
3. We do not lose our personal individual identities in some universal, generic humanity. . . .
4. Union with Christ comes to expression in, and is cultivated by, the Word and sacraments. . . .
5. The body and blood of Christ are not materially, corporeally, or physically present in the Lord’s Supper. . . .
6. In the Lord’s Supper we are lifted up by the Holy Spirit to feed on Christ. . . .
7. We are not hypostatically united to the Son. . . .
8. We are united with Christ’s person. . . .
9. It is effected and developed by the Holy Spirit through faith, in and through the means of grace: the ministry of the Word, the sacraments, and prayer (WSC 88). . . .
10. It will eventually lead to our being “like [Christ]” (1 John 3:1–2; see also Rom. 8:29–30; 2 Cor. 3:18), for “it is the intention of the gospel to make us sooner or later like God” (Calvin).

Related: Phil Gons has collected a helpful list of resources on union with Christ.