Where does the bible say anything about an immaculate conception?
The longer answer is that it's the best resolution of the Christological arguments for Christ having two natures, in one person, that were resolved at Chalcedon. The doctrine itself came about around the 6th century even though it was never formally proclaimed until quite recently. Meaning, it was seen as one of the answers but not necessarily THE answer.
It's a major, major repudiation of Nestorius, who claimed that Christ didn't truly have a human nature, and that Mary was a vessel, but not truly the Theotokos.
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