Our authority for knowing truth depends on apostolic teaching. We have no other choice than to accept their teaching; after all, they learned it from Jesus himself. But we need to discover the things they added or changed, and reject these.
The assumption is that if all the Church Fathers agree on something it must be true. Some caveats...
It is important that the Early Church Fathers address the topic because they were closest to apostolic teaching. Certain teachings only slowly became prominent over time, but once they did it seems they became nearly universal. For example, the idea of strong, hierarchical bishops became common, but this is clearly a later development, not an apostolic teaching.
Ignatius writes very early that we are to do nothing without the bishop. But he has in mind some assumptions...
Ignatius would have never expected Christians to follow heretics, or those who exploit the people or cause their faith life to languish. Yet some bishops throughout church history have been in this category.
Ignatius clearly singles out heretics to be shunned, so even he excludes certain people from being valid Church leaders. But in subsequent generations, the mere performance of the rite of ordination took precedence over the need for ordained leaders to be qualified.
The bishops Ignatius speaks of have a close relationship with those in their congregations. They often preside over the mass and other rites and rituals such as marriage ceremonies.
The teaching that Christ instituted a new kind of marriage is nonsense.
The apostles did not pass down teachings they themselves did not teach. For example, they did not pass down the doctrine of indulgences; they had no notion of such a thing. This doctrine was invented later. This does not mean God won't honor it; certainly God gives us a lot of leeway to invent devotions which he will honor. But the point is, it was not passed down from the apostles.
The apostles did not teach that certain of their statements were cultural, for example, those regarding women and hair length; so they did not pass on this notion of cultural teachings not for us today.
Certainly, Old Testament prophets sometimes uttered things they didn't fully understand which were interpreted by Jesus and the apostles. But this does not give the post-apostolic Church leaders license to interpret apostolic writings in a similar manner. The apostles were granted infallibility in their teachings; the subsequent Church leaders were not.
The questions are...
Protestants and Catholics alike agree doctrine develops, and I think it does to an extent. Certainly the statements of the Nicene Creed are doctrinal developments, as is the statement of faith of any denomination.
A further question is whether a true doctrine can contain information that the apostles didn't teach. In other words, if the apostles didn't teach it, they did not pass it on to subsequent generations, and the Holy Spirit did not intend it to be taught (because it is not true). I favor this view. Certainly we must apply apostolic teaching to new situations, but if they didn't teach something, it is not apostolic teaching.