This union is also reciprocal, as the following passages in the Word clearly show:
Philip, do you not believe that I am in the Father and the Father is in me? Believe me that I am in the Father and the Father is in me. (John 14:10, 11)
So that you will know and believe that the Father is in me and I am in the Father. (John 10:38)
So that all may be one as you, Father, are in me and I am in you. (John 17:21)
Father, all that is mine is yours, and all that is yours is mine. (John 17:10)
The union is reciprocal because no union or partnership between two exists unless each party moves closer to the other. Every partnership in the entirety of heaven, in all the world, and throughout the human form is the result of two parties moving into a closer relationship with each other until both parties intend the same things. This leads to a similarity, harmony, unanimity, and agreement in every detail between the parties. This is how our soul and our body form a partnership with each other. This is how our spirit forms a partnership with the sensory and motor organs of our body.
This is how our heart and our lungs form a partnership. This is how our will and our intellect form a partnership. This is how all our parts and organs form partnerships, both within themselves and with each other. This is how the minds of people who deeply love each other form a partnership. It is an integral part of all love and friendship. Love wants to love and it wants to be loved.
In the world, too, all things that are inseparably linked to each other have a reciprocal interaction. There is an interaction like this between the heat of the sun and the heat in a piece of wood or a rock, and in living things between their vital heat and the heat in all their tissues. There is a similar interaction between the earth and a root, through the root with the tree, and through the tree with the fruit. Also like this is the relationship of a magnet to a piece of iron, and so on.
If a given partnership is not the result of two things moving closer to each other in a mutual and reciprocal way, then a partnership develops that is only superficial rather than deep. In time, the partners in a superficial relationship drift away from each other, sometimes so far that they no longer recognize each other.
A real partnership is not possible unless it happens mutually and reciprocally. Therefore the Lord’s partnership with us is mutual and reciprocal, as is obvious from the following passages:
“Those who eat my flesh and drink my blood live in me and I in them” (John 6:56).
“Live in me and I [shall live] in you. Those who live in me and I in them bear much fruit” (John 15:4, 5).
“If any open the door, I will come in and will dine with them and they with me” (Revelation 3:20);
and other passages as well. This partnership comes about as we move closer to the Lord and the Lord moves closer to us; for it is a fixed and unchangeable law that the closer we move toward the Lord, the closer the Lord moves toward us. (For more on this, see the chapters on goodwill and faith [Sections 336–462].)
from True Christianity, Sections 99, 100