Many of the great heresies that have faced the church have come from different understandings (misunderstandings) of the doctrine of the Trinity. Wayne Grudem's Systematic Theology lists the three core components of the trinitarian belief. They are in tension, they don't "make sense" to our human mind, but deviating from any of these three core components leads us astray:
1. God exists eternally as three persons.
2. Each person is fully God.
3. There is one God.