I thought Nicea settled the Trinitarian debate for Christians, for the most part. But it seems that it has become fashionable in some circles (Evangelical and Adventist) to reject the “eternal generation of the Son.” Those who reject it claim that it leads necessarily to Arianism and subordinationism–which is ridiculous, since it was a response to Arianism. Bottom line for me: I agree with Roger E. Olson that it rests upon John 1:14. There is a relationship between the members of the Trinity that is eternal. These relationships are what allow us to distinguish between them while professing their unity. If you reject this relationality, you are left with three eternal separate beings–Tritheism.
In Adventism, this manifests itself in the idea that the members of the Trinity are interchangeably equal members of a Godhead committee. They’re running from the Scylla of Arianism only to dash themselves on the Charybdis of Tritheism.