Union with Christ
The union with Christ is a work of God whereby all believers are made one with Christ. Here two questions are to be demanded. The first, in what respect or for what cause are they said to be one with Christ? Answer....It may be said, how then are they one with Him? I answer, by one and the same Spirit dwelling in Christ and in all the members of Christ.
William Perkins, Works of William Perkins: Exposition of Galatians, eds. Shawn D. Wright and Andrew S. Ballitch, vol. 2 (Grand Rapids, MI: Reformation Heritage Books, 2019), 234