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.
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.