Some Key Terms:
ousia (oủoía) / hypostasis (inóơtaơi)

Greek: "essence" / "person" - the two key terms used by the early church to define the Trinity

 

Modalism

The heresy that God is one person appearing in three different modes - Father, Son, and Spirit as roles, not distinct persons

 

Tritheism

The heresy that Father, Son, and Spirit are three separate gods working in harmony - fails to hold the unity of God

 

The Shema

Hebrew: "Hear" - Deuteronomy 6:4, the foundational confession of Jewish and Christian monotheism