Christology is at the root of all theology. How you understand Christology determines just about everything else.
The Liberal Protestants seem to have graduated from Nestorianism and Apollinarianism into full-blown Arianism or Modalism, for example.
What we have seen on this thread is warmed over Nestorianism and Apollinarianism.
Where, for example, does it say in the Bible that the Holy Spirit is God as Macedonius argued?
Or where does it say that the Father, Son, and Holy Spirit are separate, distinct persons like Sabellius argued against in favor of Modalism?
It is fundamental also as you point out. The Reader David has referenced and quoted how who Christ is is fundamental to salvation as well. Your quote from Cyril of Alexandria is incredibly apt.
He also wrote, "I am amazed that there are some who are entirely in doubt as to whether the holy Virgin should be called Theotokos or not. For if our Lord Jesus Christ is God, how is the holy Virgin who gave Him birth, not Theotokos?" (Epistle 1, to the monks of Egypt).
Theotokos is like a canary in the mine detecting false teaching or understanding about Christ.
Thanks for your posts...