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 accept and risked their lives to oppose?
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.
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".
I believe in God the Father Almighty, Maker of heaven and earth: And in Jesus Christ his only Son our Lord, Who was conceived by the Holy Ghost, Born of the Virgin Mary, Suffered under Pontius Pilate, Was crucified, dead, and buried: He descended into hell; The third day he rose again from the dead; He ascended into heaven, And sitteth on the right hand of God the Father Almighty; From thence he shall come to judge the quick and the dead. I believe in the Holy Ghost; The holy Catholick Church; The Communion of Saints; The Forgiveness of sins; The Resurrection of the body, And the Life everlasting. Amen.
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:
Original Nicene Creed
We believe in one God, the Father Almighty, Maker of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God], Light of Light, very God of very God, begotten, not made, being of one substance with the Father; By whom all things were made [both in heaven and on earth]; Who for us men, and for our salvation, came down and was incarnate and was made man; He suffered, and the third day he rose again, ascended into heaven; From thence he shall come to judge the quick and the dead. And in the Holy Ghost.
This creed was later modified at the council of Constantinople to include the "person" of the Holy Spirit as "God".
Modified Nicene Creed From The Council Of Constantinople
We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds (æons), Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost of the Virgin Mary, and was made man; he was crucified for us under Pontius Pilate, and suffered, and was buried, and the third day he rose again, according to the Scriptures, and ascended into heaven, and sitteth on the right hand of the Father; from thence he shall come again, with glory, to judge the quick and the dead; whose kingdom shall have no end. And in the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father, who with the Father and the Son together is worshiped and glorified, who spake by the prophets. In one holy catholic and apostolic Church; we acknowledge one baptism for the remission of sins; we look for the resurrection of the dead, and the life of the world to come. Amen.
The evolution continued with the Council of Chalcedon when the "dual nature" was discussed, hammered out, and added.
Confession of Chalcedon
We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; (ἐν δύο φύσεσιν ἀσυγχύτως, ἀτρέπτως, ἀδιαιρέτως, ἀχωρίστως – in duabus naturis inconfuse, immutabiliter, indivise, inseparabiliter) the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person (prosopon) and one Subsistence (hypostasis), not parted or divided into two persons, but one and the same Son, and only begotten God (μονογενῆ Θεόν), the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
So in summary directly related to the original question:
- The Apostle's creed really doesn't add much to what we already find in scripture
- The Original Nicene creed adds Jesus to the godhead to what we find in scripture
- The modified Nicene creed adds the Holy Spirit to the godhead to what we find in scripture
- The Chalcedonian creed or Confession of Chalcedon adds the dual nature to what we find in scripture
The specific debate at the First Council of Nicaea (325) was whether Jesus Christ is like God but created by God at some early point in time, OR Jesus is God, uncreated and coeternal with the Father and the Holy Spirit. Arius was the chief proponent of the first view, so that became known as Arianism. The second view is what prevailed at the council and is expressed in the Nicene Creed.
That didn't end the debate. During the fourth century, the two sides continued to disagree; during some periods the Arians had the support of the emperor. The council at Constantinople not only expanded the section regarding the Holy Spirit, but reaffirmed the entire creed, finally settling the debate for most Christians.