What is that unity?

What is that unity?
Can we be one in thee,
As thou th’ eternal Son
Art with the Father one?
Then shall we know, when once we feel
The grace incomprehensible.

Hymnal/Album: Originally titled “That they all may be one, as thou Father art in me, and I in thee.”—[John] xvii. 21. Introduced in Charles Wesley, Short Hymns on Select Passages of the Holy Scriptures, Vol. 2 (Bristol: Farley, 1762). Published in The Poetical Works of John and Charles Wesley, Collected and Arranged by G. Osborn, Vol. 12 (London: Wesleyan-Methodist Conference Office, 1871), page 56.
Publishing: Public Domain