“I still have many things to say to you, but you cannot bear them now. When the Spirit of truth comes, he will guide you into all the truth” (John 16:12-13).
According to Jesus, the apostles were not taught all the truth. There were, in fact, many things Jesus still needed to teach them. How and when did this take place?
The how is easy. Jesus teaches the church through the Holy Spirit. The harder question is the when. Was there a time after the resurrection of Jesus that the Holy Spirit taught them all the truth?
The Council on Nicene makes it clear that the answer is no. At least up to this point truth was still being neglected. The Trinity, for example, was not explicitly taught by any of the apostles. So it can’t be true that the faith is that “which has been believed everywhere, always, by all.”
I suggest a better rule is that the truth is that which is in accordance with Scripture, Tradition, reason, and personal experience. And by reason I include science, history, and modern scholarship. In other words, the truth is that which corresponds with the evidence and is accepted by the Church Universal.
The Church, the body of Christ, determines what the truth is. Authority rests in the Church Universal, not in any individual. After all, it was the Church that created the Bible, not the Bible that created the Church.