Absolutely, as John tells us.
We can assume that the essential teaching of Christ is expressed in the Gospel; the gnostic heresy that there is or was an essentially different hidden teaching was condemned by the keeper of the totality of the revelation, the Church.
And I do thank the Church for that service to mankind. :)
Nevertheless, that leaves the commentary and the clarifications to the recorded doctrine, and that Holy Tradition was committed to writing in a non-Canonical way as the patristic legacy, the hymnody and the iconography.
I think I understand what you are saying, although I wouldn't have before this thread. :) I think you mean that dogma is not possible because we just don't have the records to prove anything. Is that close?
Not at all, -- where are your getting this? Dogma is possible precisely because the Church possesses the Holy Tradition with which it can understand the inspired Scripture. It is with Sola Scriptura that dogma is not possible.
What annalex is saying, I think, is that just as the Scriptures were not written in a vacuum, they were never interpreted in a vacuum.
The Apostles knew the fullness of the context and understanding in which the NT was written, and they passed that understanding to their followers.
While it is much more that this, Holy Tradition at the very least is the context in which the Scriptures were and are interpreted and understood.
It would never occur to devout Catholics or Orthodox to question things like the Virgin Birth of Christ or the bodily Resurrection of Christ. This is because our tradition is unequivocal on these points -- we know that there is no other way for the Scriptures to be interpreted, not if we want to call ourselves Catholics or Orthodox Christians.