I found these notes among others by Justin Taylor at The Gospel Coalition:
Union with Christ
James S. Stewart wrote that “union with Christ, rather than justification or election or eschatology, or indeed any of the other great apostolic themes, is the real clue to an understanding of Paul’s thought and experience” (A Man in Christ [Harper & Bros., 1955], vii).
John Murray wrote that “union with Christ is . . . the central truth of the whole doctrine of salvation. . . . It is not simply a phase of the application of redemption; it underlies every aspect of redemption” (Redemption—Accomplished and Applied [Eerdmans, 1955], pp. 201, 205).
Anthony Hoekema wrote that “Once you have your eyes opened to this concept of union with Christ, you will find it almost everywhere in the New Testament” (Saved by Grace[Eerdmans, 1989], 64.
If you want an introduction to the doctrine of union with Christ, John Murray’s chapter in Redemption—Accomplished and Applied is helpful, as is Anthony Hoekema’s chapter in Saved by Grace. Below are a few notes on the latter:
The New Testament uses two interchangeable expressions to describe union with Christ:
We are in Christ (2 Cor. 5:17; John 15:4, 5, 7; 1 Cor. 15:22; 2 Cor. 12:2; Gal. 3:28; Eph. 1:4, 2:10; Phil. 3:9; 1 Thess. 4:16; 1 John 4:13).
Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.
Christ is in us (Gal. 2:20; Col. 1:27; Rom. 8:10; 2 Cor. 13:5; Eph. 3:17).
20 I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
Three passages (John 6:56; John 15:4; 1 John 4:13) explicitly combine both concepts.
John 15:4 Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me.
Hoekema says that we should see union with Christ “extending all the way from eternity to eternity.” He outlines his material in this way:
The roots of union with Christ are in divine election (Eph. 1:3-4).
3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, 4 even as he chose us in him before the foundation of the world, that we should be holy and blameless before him.
The basis of union with Christ is the redemptive work of Christ.
The actual union with Christ is established with God’s people in time.
Under the third point, he shows eight ways that salvation, from beginning to end, is in Christ:
We are initially united with Christ in regeneration (Eph. 2:4-5, 10)
We appropriate and continue to live out of this union through faith (Gal. 2:20; Eph. 3:16-17).
We are justified in union with Christ (1 Cor. 1:30; 2 Cor. 5:21; Phil. 3:8-9).
We are sanctified through union with Christ (1 Cor. 1:30; John 15:4-5; Eph. 4:16; 2 Cor. 5:17).
We persevere in the life of faith in union with Christ (John 10:27-28; Rom. 8:38-39).
We are even said to die in Christ (Rom. 14:8; 1 Thess. 4:16; Rev. 14:13).
We shall be raised with Christ (Col. 3:1; 1 Cor. 15:22).
We shall be eternally glorified with Christ (Col. 3:4; 1 Thess. 4:16-17).
If we are united to Christ, then we are united to him at all points of his activity on our behalf.
in his death (we were baptized into his death),
in his resurrection (we are resurrected with Christ),
in his ascension (we have been raised with him),
in his heavenly session (we sit with him in heavenly places, so that our life is hidden with Christ in God), and we will share
in his promised return (when Christ, who is our life, appears, we also will appear with him in glory) (Rom. 6:14; Col. 2:11-12; 3:1-3).