Modalism
This understanding has been called Sabellianism and Modalistic Monarchianism. The suggestion of development and change within the Godhead was seen as contradicting the concept of impassibility. It also stood in contrast to the position of distinct persons existing within a single godhead by representing Father, Son and Spirit as different “modes” (hence the term "modalism"), “aspects” or “faces”, "masks" (persona in Latin) that God presented successively to the world. More importantly it stood against the Trinitarian teaching that "God was one God in Father" rather than One in the Father's essence only.
It has been noted also that the Greek term "homoousian", which Athanasius of Alexandria favored, was actually a term reported to be put forth and favored also by Sabellius, and was a term that many followers of Athanasius were uneasy about. Their objection to the term "homoousian" was that it was considered to be un-Scriptural, suspicious, and "of a Sabellian tendency." This was because Sabellius also considered the Father and the Son to be "one substance." Meaning that, to Sabellius, the Father and Son were "one essential Person."
Read more about this topic: Sabellius