Not at all..
Irenaeus (c. 130 – c. 202) was the first to argue that his "orthodox" position was the same faith that Jesus gave to the apostles.
Irenaeus' opponents, however, claimed to have received secret teachings from Jesus via other apostles which were not publicly known.
Ha
ha..
.. could that be that Jesus told them that there would be another prophet after him, and that he didn't die on the cross

Of course it could. Orthodox Christianity evolved over centuries. Many documents/texts were lost during the era of certain rulers.
The Nag Hamedi library is an example.
It is not "a certainty" that Orthodox Christianity is the truth, that is for SURE.
Naturally, most Christians
believe that it is. There is no hard evidence to prove it.
There was too much political wrangling, as you know.
eg. Those who possessed writings of Arius were sentenced to death
Arianism has been called the most challenging heresy in the history of the Church.
Naturally, those who agree with Arius, consider the term "Arianism" as a misnomer,
as they believe that it is much closer to the truth than classical trinitarianism, and
that Arius was not the originator of his views.