What General Councils Do

General (or Ecumenical) Councils are not innovators.

The bishops do not come together in order to think up something new out of their own minds, but in order to be witnesses of the teaching received from Christ and handed out by the Church. [Pesch, Praelectiones Dogmaticae, I:313; quoted in Hughes, A History of the General Councils, p. 17]

They do not make things up. They declare what the Church has always taught. This is not a contradiction of the principle of development of doctrine; it's a necessary part of it. As the Church's understanding grows, it becomes necessary at different times to declare clearly what in the past may perhaps have been less clear.

We can see this, for example, with the first Council of Nicaea. The Fathers assembled there did not invent the doctrine of the Trinity; rather, they formulated a way of declaring what the Church has always taught, in such a way as to distinguish it from error.

It's particularly useful by way of example to point out that the doctrine of the Trinity is by no means indisputably clear in the Bible. That was really part of the problem, because Arius appealed to Scripture in defense of his errors.

Some Protestants will say that by letting Scripture interpret Scripture, the errors of Arius are obvious. But this is special pleading. First we would have to know what portions of the Bible ought to direct our understanding of the Godhead, and that is by no means something that the Bible itself explains. It can only come from the Tradition of the Church.

For example, if Arius appeals to John 14:28 ("...the Father is greater than I"), why should we not agree that this verse is entirely clear? Why do we not let this Scripture guide our interpretation of other passages about the Godhead?

The simple fact is that there is no good reason not to do so if "sola scriptura" is the rule by which we measure things. The Bible doesn't include a key that says which are the "clear" parts and which are the "unclear" ones that must be understood in the light of the "clear" ones, and different men have different conceptions of what is clear and unclear. The only rule that can assist us here is to understand the Bible according to what the Church has always taught - which is exactly what the Fathers did at Nicaea, and what the Church teaches us today.

Read the Scripture within "the living Tradition of the whole Church" (CCC §113).

This is why Nicaea did not innovate. This is how we know that its Trinitarian interpretation of Scripture (inherited by most Protestants) is no novel thing. It was an expression of that living Tradition. This is why Arius was wrong. He interpreted the Bible contrary to the living Tradition.

Scripture, Tradition, and the Magisterium are inseparable. We need them all.