Behold, I am Jesus Christ the Son of God. I created the heavens and the earth, and all things that in them are. I was with the Father from the beginning. I am in the Father, and the Father in me; and in me hath the Father glorified his name. (3 Nephi 9:15)
What does the phrase "I am in the Father, and the Father in me" mean?
I feel like I have a better grasp on "the Father in me" part. Jesus said, "The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise" (John 5:19; see also John 8:28).
To me, Jesus seems to be describing here something more than the divine investiture of authority (wherein Jesus bears the Father's name and ministers as an authorized representative of the Father, much like someone who has power of attorney can act as the agent of another). He states that He does what he sees the Father do. I feel like, even in mortality, Jesus knew the Father so well, loved and served Him so devotedly and communed with Him so constantly that He carried the Father's very nature inside of Himself; that He could look inside to see what the Father would do in any given situation, and then He'd do accordingly.
Similarly, during His post-mortal ministry to the Nephites, Jesus prayed for His people thus,
"that they may believe in me...that they may be purified in me, that I may be in them as thou, Father, art in me, that we may be one, that I may be glorified in them" (3 Nephi 19:23,29)
His desire for us is that He might be in us as the Father is in Him. We bear His name as He bears the Father's. And He invites us to believe and be purified in Him, to know, love and serve Him so devotedly and to pray so constantly that we receive the image of Christ in our countenance, that His light shines from us, and that we can look inside ourselves to know what Jesus would do in any given situation, and then do accordingly.
But how is the Son inside the Father?
The best way I can try to understand it is to lean on the parallelism described in John 17:21, "That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me."
That would seem to imply that as Jesus is in the Father, so am I (or can I) be in Jesus. And suddenly it strikes me that I must be. How could He literally take upon Him all my pains, frailties and struggles and I not become a part of Him? His love of me is so deep, He says, "I have graven thee on the palms of my hands. Thy walls are continually before me" (Isaiah 49:16).
Is Jesus in the Father in the same way? We know so much about the Son's devotion to the Father, and have so little detail about the Father's devotion to the Son. But again, if I lean on the parallelism in John, if the Son is in the Father the same way that I am in the Son, and if the Father's capacity is greater than was the Son's in Gethsemane, then that devotion is staggering in its immensity.
I love the implications of this for the concept of unity. For Jesus, part of being one with the Father meant subjecting His will to the Father's, just as part of becoming one with Jesus means that I need to subject my will to His. But that doesn't mean that the Son's will got lost in the Father's, because He was also in the Father. The Father's will, then, included the Son's. Maybe that's why Abinadi talks about "the will of the Son being swallowed up in the will of the Father" (Mosiah 15:7); it didn't get eradicated but incorporated into something even bigger, brighter and more beautiful.
So, when I subject my will to the Lord's, my will doesn't get lost either, because I am in Jesus too and Jesus is in the Father. Both know my every pain and my dearest hopes, internally. I can never stop being precious to Them, and They will move heaven and earth for my self-actualization. Every sacrifice I make to be one with God is not, in the end, a loss to me, but a gain, because it creates a passage for Him to become more and more in me, like I already am in Him.
Right now, I can't comprehend being one with God like Jesus is one with the Father. The devotion shown on both sides of that relationship is so far beyond my capacity and understanding that I'm intimidated, maybe even terrified, by what it would take in terms of sacrifice for me to develop it. But I don't think total devotion requires for me to give more than my capacity. Just the fulness of my capacity (which will then be ever-increasing). And right now, I just need to focus on loving God a little better today than I did yesterday. That's not scary. That's safety. I can trust God to help me let Him in, to teach me to love Him like He loves me.