There were opposing views especially on the nature of God and His relationship to Jesus Christ and the Holy Ghost. That was in part the reason for the councils. To try to unify the church and come to some form of consensus on the many points of doctrine that were in conflict. I would assume that the discussions were similar in nature to the ones we are having here on this site. What is the Biblical basis for disbelief in the doctrine of the Trinity? and... Biblical basis for the doctrine of the Trinity Both sides using the scriptures in support of their beliefs. Some how they were able to come to some sort of consensus and the resulting creeds were deemed infallible by the church. Those who disagreed with them were often excommunicated and even exiled. That is the reason for my question. What did the Non-Trinitarians believe was being added to the concept of God by these creeds that they could not except and risked their lives to oppose?
The biggest thing about these councils specifically is the addition of God being three persons, which "persons" were to be included in the term "God", and the evolution of such. For example, the Apostle's Creed which pre-dates the first of these councils (Nicea) says nothing about God being more than one nor anything about multiple "persons" being "God".
However, the Council of Nicea took it a step further by making God and Jesus as "persons" of "God", but not the Holy Spirit. The Holy Spirit in the original Nicene Creed was simply tacked on the end so as to be included. Observe:
This creed was later modified at the council of Constantinople to include the "person" of the Holy Spirit as "God".
The evolution continued with the Council of Chalcedon when the "dual nature" was discussed, hammered out, and added.
So in summary directly related to the original question:
When framed in terms of what can't be found in the Bible: Almost nothing can't be found in the Bible / You can find almost anything in the Bible. A wide variety of established beliefs are "Biblical" because, like most any non-technical document, it's vastly multi-interpretable. (And even technical language is multi-interpretable.)
The creeds primarily clarify which interpretations were presumably authoritative [and correct].
Taking a "simple" example, in John 10:30, Jesus says, "The Father and I are one." That could mean anything from Jesus really "digs" the Father to Jesus shares Father's mission to Jesus and the Father are the same essence and being. Not to mention the gnostic understanding, which may have interpreted the statement as an indication of the godliness of "any enlightened person."
The creeds document in technical language the beliefs that the Biblical texts were written to illustrate, among them a belief in a Trinity God of co-equal, co-eternal Persons. The range of Non-Trinitarian and Trinitarian beliefs is vast. Many believes can be pretty convincingly presented as "Biblical." The key point here is that scripture was written to assist with the promulgation and poetry surrounding existing beliefs. Widespread Misinterpretation of that scripture, amongst other ambiguities and "belief drift", was largely the fuel for firing up the councils and documenting the intended beliefs in more technical language.
And while each council had some foci, some specified heresies to address, there just a lot of variation in belief that scripture cannot in itself pass judgement on.