"When all things are subjected to him [the Son], then the Son himself will also be subjected to him who put all things in subjection under him, that God may be all in all." (1 Corinthians 15:28 ESV)
I had a good discussion recently with our guest speaker on the following statement. The context was Hebrews 1:2, Philippians 2, and the emptying of Christ at his incarnation...
"Once he surrendered that place as divine Spirit beside the eternal Father, once he became human, there was no turning back. He could never go back to what he was before. He had become human; he couldn't 'un-human' himself. He could never again be divine Spirit. He gave up that position. No man, not even Jesus Christ, can be equal with God."
Now we had a good discussion on this, and what he was saying was that Jesus laid aside his divine status and united himself permanently with a human nature. And so, he would forever as the God-man represent the human race before God. As the human Representative he therefore could not have equal status with God in that character and role.
There's much with his statement I would agree with, but I would qualify it at two points.
1) Though after his Incarnation the Lord Jesus could never again be Divine Spirit, he would never cease to be a Divine Person. He never gave up his Godhood, but rather laid aside his privileges as God and joined himself with humanity by fully taking on human nature. He laid aside his glory and his divine privileges, but he did not divest himself of them.
2) For Jesus to submit himself to the Father does not imply inequality. His nature as divine Son is eternal and unchanging, before and after the incarnation. At the incarnation he took "the form of a servant... in human form" (Phil 2:7-8). But he has been forever the Son. And in this relationship there is both submission and equality:
"...he was even calling God his own Father, making himself equal with God. So Jesus said to them, 'Truly, truly, I say to you, the Son can do nothing of his own accord, but only what he sees the Father doing. For whatever the Father does, that the Son does likewise.'" (John 5:18-19 ESV)
As our divine-human Mediator Jesus would say and do things that highlighted either of his two natures, divine and human. These natures can be distinguished but not divided from his glorious Person. They are not separable, nor intermingled (like being a half-God-half-man kind of person).
He is both God and man in one person. As to his divine nature he can say "I and the Father are one" (John 10:30), "whoever has seen me has seen the Father." (John 14:9), and he receives worship (Jn 20:28; Rev 5:12-14). As to his human nature he can say, "the Father is greater than I" (John 14:28), "I thirst" (Jn 19:28), and "concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only." (Matthew 24:36).
So, is Jesus equal with the Father? Ever since the Arian controversy of the fourth century Christian churches have largely held that the Father and Son (along with the Spirit) are co-equal as to their shared essence of Godhood (Deity, God-ness). One eternal Being, God, co-existing in three Persons.
The Nicene Creed (AD 325) stated, "And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds; God of God, Light of Light, very God of very God; begotten, not made, being of one substance with the Father, by whom all things were made."
Athanasius, a key character at the Council of Nicaea, put it this way: "And in this Trinity none is before or after another; none is greater or less than another. But the whole three persons are coeternal, and coequal." (Athanasian Creed)
Christ is equal with the Father in essence, majesty, attributes, and eternality. The church debated for many years after that, seeking to understand just how the two natures (human + divine) could exist in the one person, Jesus Christ. The Chalcedonian Creed (AD 451) put it this way:
"Our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a rational soul and body; coessential [of one substance] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin... one and the same Christ, Son, Lord, Only-begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ..."
The Son of God is not a created being (as Arius believed) but resides in the bosom of the Father from eternity past, and to eternity future! Christ in his Deity remains unchangeably divine, whether before or after his Incarnation. So God did not become man in some transmutational way, or with the loss of divinity itself. Rather, the second Person of the Triune God joined himself permanently to humanity by taking on a human nature. So, it is true that Jesus the God-man will never un-human himself. But neither will he un-deify himself!
And yet this does not mean that all Persons within the Triune Godhead have the same roles. If we mean "equality" in the sense of "same as" or "same role as", then that statement would have to be qualified. It appears that the hierarchy (authority and submission) which we see in Christ's life (e.g., love and submission toward his Father) has existed (and will exist) within the Trinity forever. The Father sends the Son (before the incarnation), the Son accomplishes the Father's will, the Father and Son send the Spirit. The Spirit glorifies the Son, the Son glorified the Father, who then glorifies the Son...! So at the end the Son "subjects himself" to the Father (1 Corinthians 15:28), which is not something he does merely because he has a human nature, but appears to reflect essential relationships within the Trinity.
Finally, I believe it's important how and what we think about Christ, since he is the One we trust for salvation, and our Example of Sonship that we should follow. (So ably portrayed by our guest speaker!) We should understand Christ as clearly as we can, as he is revealed in the Scriptures. The challenge of theology (for example, the Trinity) is not to come up with a statement that is supported by a few verses, but a statement that encompasses and explains all that the Bible has to say about that topic.
At this point, we must just marvel and worship. There are so many unfathomable mysteries here!
Suggested for further reading:
The Athanasian Creed
Athanasius' On the Incarnation
What is the Trinity, a free eBook by R. C. Sproul
Delighting in the Trinity, by Michael Reeves.