The council that met at Nicaea in 325 was called by the emperor Constantine rather than by a bishop, and that is the first thing about it: an imperial summons, imperial post horses to bring the bishops, and an imperial residence to sit in. The question before it was whether the Son is of the same substance as the Father or a creature, however exalted, made before time.
Arius, a presbyter of Alexandria, held the second view and had the support of several eastern bishops; the council decided against him and wrote the word homoousios, of one substance, into its creed, a term that appears nowhere in scripture and that a good many of those who signed disliked. It also fixed a common rule for calculating Easter so the churches would not celebrate the resurrection on different days, and issued twenty canons on clerical discipline and on the readmission of those who had lapsed under persecution. What the council did not do was settle the argument.
Arian and semi-Arian parties held imperial favour for much of the following half century, and the creed recited in churches today is not the text of 325 but a longer form associated with the council of Constantinople in 381. The authority of Nicaea was established retrospectively, by the councils that appealed back to it.