Pretty much so, FK, with only a few decades separating the two events. The Creed was finalized at the First Council of Constantinople (Second Ecumenical Council), only about a decade before the canonization of the Bible in the west.
But I thought that you all use circa 33 A.D. as the starting point of your particular church. Wouldn't that mean that everyone had it wrong for 300+ years? Wouldn't that also mean that either the Apostles were teaching different things or that they were terrible teachers? :)
Moreover, extant Bibles dating from circa middle of the 4th century contain books that are no longer considered canonical. Any copies of the New Testament books prior to the Nicene Council are mysteriously missing (presumably destroyed by the Church), but fragments and indirect evidence shows that the post-Nicene sources have been heavily redacted to more closely reflect the new trinitarian dogma of the Church.
Do you infer from that that the original manuscripts were altered to match a man-made theology? If so, then the Bible would really be of no spiritual value. It would be no better than, say, Dianetics. :)
This is why I always tell you that blind faith comes before theology. In other words, people proclaim belief in Christ before even knowing why they believe! When a whole bunch of people were "licked" by the fire-like Holy Spirit on the Pentecost, none of them knew why they believed; they were suddenly transformed, according to the Bible, from non-believers into believers. I think it is safe to say that the "multitudes" acknowledged Jesus as the Christ, but couldn't really tell you much more what that entailed.
They knew nothing about the faith, pretty much like infants. In fact, baptism was seen as the substitute for circumcision, so there is no reason why the earliest Christians, who were mostly Jews, would not baptize children, since all Jewish infants are circumcised shortly after birth.
But, as time went on, Christians began to realize that what they believed in was slightly different for the guy next door, so they began to congregate into different groups, according to like-mindedness, insisting they were right and others were wrong. In doing so, they had to define and defend what they believed in, thereby giving rise to specific Christian theologies.
This differentiation apparently took place fairly soon, because the book of Acts and Paul speak of such rival groups, and by the middle of the second century these divisions blossomed into full blown theological wars. The religious party we otherwise know as the "orthodox" side was advancing its beliefs, and won in establishing itself as the dominant Christian theology, but it took three hundred years to do so, and even then not in an absolute sense.
Like any party of men, it needed bylaws (canons) and statements of faith (corporations call them mission or vision statements), and the ecclesial (church) administration mechanism along with officers (hierarchy) to run it. It was no longer an ekklesia (i.e. simply gathering of the faithful), but a well-defined organization with an equally well-defined set of beliefs (theology).
The same can be said of modern Judaism being the outgrowth of Pharisaical Judaism. The Essenes and the Sadducee religious parties vanished (there are only about 700 Samaritans left, so they don't count, although they are the only genuine surviving priestly Judaic sect with animal sacrifices to this day because their temple was never destroyed).
The surviving (redefined) rabbinical Judaism then began to splinter in a fashion similar to that of Christianity according to differences in how different people perceive their faith.
So, yes, among the "multitudes" who were converted on the Pentecost, there were those whose faith was of the orthodox variety, even if they didn't have it all neatly defined and written down.
There is ample evidence of alterations as time progressed. As for the fate of the original documents, let's just say they were all "lost" (conveniently). What we are left with is the orthodox theology and the orthodox canon (in that order) that have been carefully preserved by the orthodox Church (I am using orthodox here as a party name or designation, such as republican, progressive, etc., and not as meaning "right").
This is probably not what most Orthodox/Catholic believers want to hear, but evidence shows that this is the case. The Church has actively engaged dissenters first with arguments and later with other means, and in active destruction of writings that disagreed with orthodoxy, for the sole purpose of removing any impurity from the mutually agreed orthodox version of Christianity.