Here's a breakdown:
* Early Church Fathers: While not using the term "Hypostatic Union," early Church figures like Athanasius of Alexandria (c. 296-373) and Cyril of Alexandria (c. 376-444) laid the groundwork by arguing for the divinity of Christ and his full humanity, but unified in one person. They emphasized the "two natures" of Christ (divine and human) in one "hypostasis" (person).
* Council of Chalcedon (451): This Council is widely considered the pivotal point in defining the doctrine. The Council's "Definition of Faith" affirmed Christ's two natures (divine and human) "without confusion, without change, without division, without separation," existing in one "hypostasis" (person). This became a foundational statement for understanding the Hypostatic Union.
* Later Development: The term "Hypostatic Union" itself gained prominence in the 6th century, with theologians like John of Damascus (c. 675-749) employing it more explicitly.
In essence, the Hypostatic Union wasn't adopted in a single year, but emerged over time through theological discussions, council pronouncements, and the writings of key thinkers. It stands as a central doctrine in Christian theology, particularly within the Eastern Orthodox and Catholic traditions.