The existence of heretics proves there was never a complete consensus; Fr. Thomas Hopko has said that there has never been complete agreement in the Church, and even as early as the Council of Jerusalem, we have Peter and Paul arguing somewhat heatedly.
I see, so there was never complete agreement.
The Church has always recognized the authority of the Apostles, who established the Tradition of meeting in Council and of their successors found in the episcopate of the Church. We have all, universally (Orthodox and heretic) believed that the Holy Spirit guides the Church and that the gates of hell will never prevail against the Church.
Yes but what is to say that the traditions affirmed at the Ecumenical Councils were the original teachings of the Church?
We see one strand of Christianity, Orthodoxy, that has existed for 2,000 years while all others have fallen by the wayside. "branches" of the Church such as Arianism have shown to be heretical, they do not bear fruit and therefore are cut off and burned. These doctrines are condemned by the Holy Spirit through the work of the Church, and their branch withers. This is the testament to the truth of Orthodoxy against the other forms of early Christianity such as Gnosticism, Docetism, Adoptionism, Montanism and Arianism.
Even still, how do we know that the traditions of the Church are the original teachings? Do we have records back to the earliest years?
It is called Arianism because it's origin can be identified with Arius, an Alexandrian presbyter at the start of the 4th century.
Now, Arius and his followers claimed that their interpretation of Scripture was the 'true one'. But that was the whole reason the First Ecumenical Council was called. The bishops, the successors of the apostles, from all over the Empire, representing what had been transmitted in Rome and Alexandria and Antioch and Greece and so on and so on, were called together to determine whether Arianism was consistent with the tradition (including the traditional understanding of the Scriptures) that had been passed down from the apostles in their churches. And their unanimous witness was that Arianism was not the Gospel taught by the catholic Church, but was something different than what the Apostles had passed down (and therefore, per St. Paul, anathema).
So it did not appear near the beginning of Church history but only later on?
The trinitarian position on Christ is that He is divine with the same divinity as the Father, and pre-existed as the Logos, who created all things.
The Arian position on Christ is that He is divine with similar divinity as the Father, and pre-existed as the Logos, who created all things except for himself.
I'm pointing this out because many critics of Orthodoxy do not understand how very similar Arianism is to Orthodoxy, and how reliant it is upon the same neoplatonic base, which explains why it arose in the fourth century and not the first. Arianism is not some cool exotic other version of Christianity.
What is important to know is if the tradition upheld by the Orthodox Church is the same as that of the early Church. How can we know if it is the case? Even if the Bible was written by the Church there are apparently some elements of it based on sources from the time of Christ. This therefore challenges to some degree the claim that the Bible belongs to the Church alone.