Sabellianism is a theological belief that emerged in the early Christian church, named after the theologian Sabellius. It posits that the Father, Son, and Holy Spirit are not distinct persons but rather different modes or aspects of one God. This view emphasizes the unity of God over the traditional understanding of the Trinity. The doctrine was considered heretical by many early church leaders, as it challenges the orthodox view of the Trinity, which maintains that the Father, Son, and Holy Spirit are three distinct persons in one divine essence. Sabellianism highlights the ongoing debates about the nature of God in early Christianity.