Like many other heresies, this week's heresy attempts to use human reason to understand the divine. That which seems illogical to us would very likely make more sense if we had a grasp of infinite knowledge, but since we don't, many like to try and understand that which we simply can't--and that need to "know" often leads to the creation of heresies.
Nestorianism is a 5th century heresy that states there is no union or intermixing or touching of the two natures of Christ. This was declared a heresy at both the 431 First Council of Ephesus and the 451 Council of Chalcedon. Some individual churches broke off after this to form the Church of the East (not the same as the Eastern Orthodox Church, also known as the Assyrian Church). Nestorianism is seen as the antithesis of Monophysitism. Nestorianism teaches that Jesus and the Christ are two separate beings, loosely united in one nature: human. This was a heretical rational attempt to explain and understand the incarnation of the Divine into humanity. He also challenged the title of “Theotokos” (“bringer forth of God”) for the Virgin Mary, saying such a title denies Jesus’ full humanity. His suggested replacement was “Christotokos” (“bringer forth of Christ”). Nestorian opponents viewed this as an Adoptionism-like heresy. Nestorianism survived well into the 14th century.