Sabellians, so called after Sabellius, a theologian of the early 3rd century, were heretics belonging to the school known as the Monarchians because they held only one Divine principle in the Trinity.
The Sabellians first taught that the Father became Man in Christ and gave His life for the redemption of the world.
They favored an essential and numerical identity between the Father and the Son.
Later they appear to have modified their doctrines.
According to Epiphanius they maintained that just as three realities go to constitute man-body, soul, and spirit, so in God three realities constitute One Person; but these realities are so explained as to mean three modes of acting or manifestations.
Sabellianism was the popular name for this heresy in the East; in the West it was more familiarly known as Patripassianism (Latin: pater, father; passus [pati], having suffered).
Repeatedly condemned, it did not die out until the 5th century.
New Catholic Dictionary