(This is what John Brown of Haddington says about the metaphors for the believer’s union with Christ.)
In attesting the reality of this union between Christ and believers, the Scripture represents him as in them, and them as in him (John 14:20, John 6:56, John 15:4, 5, 7, John 17: 21, 26, Colossians 1:27, 1 John 5:20, 2 Corinthians 5:21, Isaiah 14:17); and having him for their life (1 John 5:11, 12, Galatians 2:20, Colossians 3:3-4); and being partakers of him (Hebrews 3:14).
This spiritual union between Christ and believers, being exceedingly mysterious in itself, is in Scripture illustrated to us by many similitudes, some of which transcend it, and others are transcended by it.
1. It is likened to that union which is between the persons of the Godhead (John 17:21, John 14:20, John 6:57). But here it falls infinitely short, not being absolutely necessary, or self-existent; nor doth it constitute Christ and believers one individual substance.
2. It is likened to the union of Christ’s two natures in his person. For as his manhood was conceived by the power of the Holy Ghost, we are born of the Spirit (Matthew 1:20, Luke 1:35, John 3:5, 6, 8, 1 Peter 1:3, 23, 1 John 3:9, 1 John 5:18) As Christ, by a sovereign act, assumed our nature, he by another apprehends our person (Hebrews 2:14, 16, Philippians 3:12). As in his manhood dwells all the fulness of Godhead, we, being in him, are filled with all the fulness of God (Colossians 3:9, 10, Ephesians 3:19). He, being made flesh, tabernacled with us, and we, being united to him, God dwells with us in him (John 1:14, Revelation 2:13, Ephesians 2:21-22, Ephesians 3:17). In him, as God-man, there is the grace of union, unction, and headship; and in us, as united to him, there is a gracious union, unction, and membership (John 1:14, 16, Colossians 2:19, Colossians 1:18).
Nevertheless, our spiritual union with him falls far short of the union of his two natures, as it doth not render him and us one person, nor, for a time, incapable of sin (Galatians 5:17, Romans 7:14-25, Romans 8:13). But it is indeed by that new nature which his self-uniting act forms in us that he holds fellowship with our soul (2 Peter 1:4, 2 Corinthians 5:17, Galatians 6:15); and which, by his gracious influence, mortifies our inward corruption, till it be utterly abolished (Romans 8:2, 13, Galatians 5:17, 24, Romans 7:14-25).
3. It is likened to the union between a king and his subjects, because he, as our brother, hath power over, cares for, rules, and protects us; and we are voluntarily subject to him, and have our eternal happiness dependent on his infinite wisdom, power, mercy, and honour (Revelation 15: 3, Matthew 25:34-40). But it is much more spiritual, close, and permanent.
4. As it imports mutual knowledge, choosing, and solemn self-dedication, and issues in mutual love, delight, and interest, it is likened to the marriage-union betwixt husband and wife (Ephesians 5:30, 32, Isaiah 54:5, Ezekiel 16:8-14, Song 2:16, Song 6:3). But here also it much transcends, as it renders Christ and believers one spirit, and can never be dissolved (1 Corinthians 6:16, 17, Philippians 2:5, 2 Peter 1:4, Colossians 3:3, Hosea 2:19-20).
5. To mark that their happy connections, support, and glory, depend on him, it is likened to the union of a building with its foundation or corner-stone (Isaiah 28:16, 1 Corinthians 3:9, 11, 17, Psalm 118:22, 1 Peter 2:4-5, Ephesians 2:20-22). But here also it far transcends, as Christ is equally near to every believer, and communicates life to every believer (1 Peter 2:5, Galatians 2:20, John 14:19, John 11:25).
6. Because through it we receive all our supporting, quickening, beautifying, and fructifying influences, it is likened to the union between the root of a tree and its branches (John 15:1-7, Colossians 2:7). But here also it far transcends, as Christ, our root, is equally near to all his branches, and not one of them can become altogether withered, barren, or broken off (Romans 7:4, Romans 6:14, Romans 8:35-39, John 10:28-29).
7. As we are enlightened, governed, honoured, and receive our spiritual, nourishment and breath through Christ, it is likened to the union between our head and other members of our body (Ephesians 4:15-16, 1 Corinthians 1:12, Colossians 1:18, Colossians 2:18-19). But it far transcends this, as Christ is equally near to every member, and none can be separated from him, or become utterly benumbed or mortified (John 14:16, 19, Colossians 3:3-4, Galatians 2:20, Isaiah 26:19).
8. As Christ enters into our soul, and is the very life of it, our spiritual union with him is likened to that of our soul, or of our food with our body (John 6:56-57, Colossians 3:4). But it is much more close, as Christ can never be separated from us, or cease to actuate us (Ephesians 4:16, Colossians 2:19, Galatians 2:28).
This union is formed in the work of effectual calling, in which Christ, by his Word and Spirit, invites, drives, and draws them to himself; and, in his powerfully applied declarations and offers of the gospel, conveys himself and his grace into their hearts. This effectual calling is the work of God (Romans 9:24, Romans 8:20, Romans 11:29, 1 Thessalonians 4:7); and is ascribed to the Father (1 Corinthians 1:9, 2 Timothy 1:9); and to the Son (Romans 1:6, 2 Peter 1:3); but, in a peculiar manner, to the Holy Ghost, as sent by the Father and Son to apply redemption to us (Romans 8:2, 2 Corinthians 3:6, Revelation 2:7, John 16:7-13, Ezekiel 36:26-27, Isaiah 44:3-5).