At the Council of Nicea it was homoousios that was at issue. Is the Son "one substance" with the Father?
If the Son was the Word inside of the Father before his begetting in eternity past, then he is one substance with the Father. In other words, he's made of the same divine "stuff," "substance," "material," or "essence" as the Father because he came out of the Father.
If he was created from nothing, then God must have used something to create him; probably whatever he created the angels with. Greek Christians simply called that substance "matter." If that's what the Son was made from, the Nicene council reasoned, then he's not really God.
Athenagoras, a 2nd century apologist, addressed the issue of matter and God long before Nicea, claiming to speak for all Christians:
He makes it clear that Christians would never worship matter. But he has also already said that they do worship the Logos of God.
The doctrine of Arius, that the Son was created from nothing, made him matter and not the substance of God. Arianism denied homoousios.
The Nicene Creed answers Arianism directly. If you already read the page on the Logos of God, then you should be able to understand exactly what's being
The Son, because he is the Logos of God, and always was the Logos of God, is of the Divine substance. Therefore he is "God from God … true God from true God."