The Last Adam

When all things are subjected to him, 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 Cor 15:28).

That' a unitarian prooftext. A few observations:

i) This occurs in the same letter where Paul uses a divine title for Christ: "The Lord of glory" (1 Cor 2:8). That's a variation on a divine title (e.g. Ps 24:7-10; Ps 29:3; Eph 1:17). The Kurios of glory is the Greek equivalent of the Yahweh of glory.

ii) This occurs in the same letter where Paul adapts the Shema (Deut 6:4) to make Jesus the Lord (=Yahweh) of the Shema (1 Cor 8:6). Yes, I know that gives unitarians heart palpitations, but it's unmistakably what's happening in 1 Cor 8:4-6). 

iii) God assumes a number of economic roles, viz. creator, redeemer, sustainer, judge. These are contingent roles. They are inessential to what God is in the sense that while only God can discharge those roles, God would still be God if he didn't undertake them.

iv) Likewise, social roles can either be temporary or permanent. If a man is widowed and remarries, he's no longer the husband of his first wife. That role was temporary. By contrast, if he fathers a child, that role is permanent.

But again, even on human terms, these are contingent roles or relations. It's essential to human nature that we have the capacity to be a spouse or parent, but inessential that it be realized. We'd still be human without it.

v) In addition, it's possible to have multiple roles which vary in authority or social status. The same individual can be equal or superior in one role, but unequal or subordinate in another role. For instance, a grown son who's a successful businessman might hire his own father. His father is his son's employee. In that respect, a subordinate. But that doesn't mean the son outranks the father in every respect. 

vi) As one scholar documents, in 1 Cor 15, Paul is using Adam typology for Christ. That's implicit in v27, where he quotes Ps 8:6–but explicit in v45, where he designates Jesus as the Last Adam. Cf. G. Beale, A New Testament Biblical Theology (Baker 2011), 261-62. 

So that's an economic role, grounded in the humanity rather than the deity of Christ. The Son qua Last Adam is eternally subordinate to God, but the Son qua Son is on a par with the Father, as the Yahweh of glory and the Yahweh of the Shema (see above). 

Put another way, the Son in his humanity is naturally subordinate to God. But that's not all there is to Jesus in Pauline Christology. The inequality of the Son qua Last Adam is consistent with the coequality of the Son qua Son.