Q. 1. What can you tell me about the heresy of "Sabellianism?"
A. 1. The heresy of "Sabellianism," (also known as modalism, modalistic monarchianism, or modal monarchism) is a denial of the Trinity as three distinct persons within the Godhead. It promotes that the Heavenly Father, Son and Holy Spirit are different modes or aspects of one God.
"The term Sabellianism comes from Sabellius, a theologian and priest from the 3rd century. Modalism differs from Unitarianism by accepting the Nicean doctrine that Jesus is fully God." (wikipedia.org)
This heresy was originally stated by Noetus of Smyrna c.190. It was refined by Sabellius c.210 who applied the names merely to different roles of God in the history and economy of salvation.
Noetus was condemned by the presbyters of Smyrna. Tertullian wrote Adversus Praxeam against this tendency and Sabellius was condemned by Pope Callistus.