Adoptionism
Adoptionism is the belief that Christ was originally just a man, who entered a unique union with God.
"Christ is mere man."
Theodotus the Cobbler, late 100's CE, reported by Eusebius of Caesarea in Ecclesiastical History 5:28:6, c.323 CE.
Filionaturalism is the belief that Christ was God from the beginning.
"We believe ... in one lord, Jesus Christ, ... of the essence of the father, god of god, light of light, very god of very god, begotten not made, coessential with the father."
Creed of Nicea, 325 CE.
Eusebius says that Theodotus was the first to affirm Adoptionism, but it seems not to have been addressed until it, or something similar, was revived in the 300's CE, for instance by Photinus of Sirmium.
"He whom one holds to be [a] god can by no means be brought into the womb."
Photinus of Sirmium, reported in a 362 CE Epistle by Flavius Claudius Julianus ("Julian the Apostate"). (Brackets added. The "a" in the translation I cite cannot literally be from the original, and it strikes me as odd since presumably the author at least affected monothesism. But I have neither seen the original, nor inquired into the thinking of the translator, doubtless a more accomplished Classical scholar than myself, so maybe "a" accurately conveys Photius' intention. I merely reserve judgment.)
In its original form, Adoptionism (as its name indicates) affirmed that while originally mere man Christ became in some unique sense God's son. Some subsequent forms have dropped this affirmation, including Theological Liberalisms such as Unitarianism. But they are still forms of the same heresy, for once filionaturalism is denied the rest is detail.