We don’t hear a lot about EFS these days online, but a couple of years ago it was all the rage. Part and parcel with eternal functional subordination is a social trinitarianism wherein we seemingly have three subjects, not one in the Divine Monarxia. I note a social trinitarianism inherent to the EFS position precisely at the point that we can ostensibly think that the Son could somehow be eternally obedient to the Father in a ‘subordinate’ manner. This reeks with the notion that there is a rupture between the Father and the Son (and thus by extrapolation, the Holy Spirit) such that the Son’s being is distinct from the Father’s; just at the point that it’s conceivable that the Son could in any way be subordinate to the Father in the eternal reality. What we have then, if this is the case is a tri-theism; viz. the idea that there are three distinct centers of consciousness within the Godhead. But as orthodox Christians we know this is nothing more than a load of piping hot rubbish.
Karl Barth, even though he enjoys some level of whipping boy status among the retrievers of Post Reformation Reformed orthodox theology, is in line, in his own lexical way, with the catholic and Protestant Reformed thinking on the single subject reality of God’s eternal being as Father, Son, and Holy Spirit (some like to refer to this as God’s simplicity). In other words, Barth serves as an excellent voice of clarity contra EFS, subordinationism (of all stripes), and tri-theism. Here he writes on the complicated lexical reality of ‘person’ as that relates to the unity of being as the Divine life; unity in reference to the multiplicity of persons as the simplicity of the eternal living God.
“Person” as used in the Church doctrine of the Trinity bears no direct relation to personality. The meaning of the doctrine is not, then, that there are three personalities in God. This would be the worst and most extreme expression of tritheism, against which we must be on guard at this stage. The doctrine of the personality of God is, of course, connected with that of the Trinity to the extent that, in a way yet to be shown, the trinitarian repetitions of the knowledge of the lordship of God radically prevent the divine He, or rather Thou, from becoming in any respect an It. But in it we are speaking not of the three divine I’s, but thrice of the one divine I. The concept of equality of essence or substance (…, consubstantialitas) in the Father, Son and Spirit is thus at every point to be understood also and primarily in the sense of identity of substance. Identity of substance implies the equality of substance of “the persons.”
This is hard teaching for those committed to sub-orthodox machinations, but it is the better way. For orthodox Christians there is One God in Three Persons, and Three Persons as the One God. The best way is to simply allow this biblical and revelatory reality to contradict any other sort of runaway ideas you might have about God; even with the best of your intentions in tow. Christians are Trinitarian Monotheists; don’t allow the Trinitarian part to throw you off though. We hold to simplicity (properly reified) in multiplicity; in this we don’t confuse ousia (being) with hypostaseis (persons), instead we hold them together in just the way they have been revealed in the singular name of the living God.
 Karl Barth, Church Dogmatics I/1§9, 56.