In the early fourth century Alexander, Athanasius, and others began teaching the Father and Son are coeternal and coequal.
Arius, Eusebius, and others rejected the Son’s eternity because he was begotten and thus had a beginning.
In 325, Constantine called the council of Nicea, which produced a new creed saying the Father and Son were of the same substance (homoousios).
Nicea did not settle the matter. Christians battled from 318 to 381 over whether the Son was eternal.
Logos incarnationists like Eusebius and dynamic monarchians like Photinus fought back, but lost to the Nicenes led by Athanasius.
In 381, emperor Theodosius called the council of Constantinople, which produced a new creed in agreement with Nicea, while also including the Holy Spirit.