Indwelling, Union, and Unity

Thomas Boston once wrote that "the gospel is a doctrine of mysteries," the result of which is that we are a mystery to the world and even a mystery to ourselves:

O what mysteries are here!

The Head in heaven, the members on earth, yet really united!

Christ in the believer, living in him, walking in him: and the believer dwelling in God, putting on the Lord Jesus, eating His flesh and drinking His blood!

—Thomas Boston, Human Nature in its Fourfold State (reprint, Banner of Truth Trust, 1989), 257. HT: Marcus Johnson

One of the best places to see these multiple indwellings—including within the Trinity—can be found in Jesus' high priestly prayer of John 17.

The Father is in the Son (John 17:21, 23)
The Son is in the Father (John 17:21)
Believers are in the Father and the Son (John 17:21)
The Son is in believers (John 17:23, 26)

Sketch it out and it looks something like this:

This prayer alone is worth a lifetime of reflection.