There are three mysterious unions in our religion. (1.) The substantial union of the three persons in one Godhead. (2.) The personal union of the divine and human natures in Jesus Christ. (3.) The mystical union betwixt Christ and believers, which is that wherein Christ and believers, are so joined, that they are one Spirit, and one mystical body, 1 Cor. 6:17 and 12:13.
The Whole Works of Thomas Boston: An Illustration of the Doctrines of the Christian Religion, Part 1, ed. Samuel M‘Millan, vol. 1 (Aberdeen: George and Robert King, 1848), 546.