Even if God is the Creator why is he a trustworthy source?[/quote
God and the Creator are the same being,
alang. LOL! God = Creator. Creator = God.
Because he says so? Maybe he is mistaken and a Creator made him?
No,
alang. Logic tells us that a being of eternal self-subsistence exists. I don't know precisely who you think this created being you go on about is, but he ain't God. God by definition did not begin to exist and is, therefore, not a creature.
We can only assume they were written then since our oldest Biblical text fragments date to the 2nd century. These fragments we have don't match exactly with other fragments leading scholars, not just Ehrman, to believe the texts have been changed over time. It is likely that Mark 16:9-20, coincidently covering the resurrection, is such an addition.
Actually, via the forensics of textual criticism regarding the most textually documented work of ancient history, namely, the New Testament, scholars can confidently affirm the reliability of the oldest surviving codices back to the first century. Understanding why that's so requires knowledge beyond the scope of this discussion. But the following gets you on your way, though there is so much more to know:
Were the New Testament Manuscripts Copied Accurately?
From the oldest surviving fragments and codices of the New Testament in Greek, Latin and other languages, literally thousands of manuscripts, and also from the oldest surviving codices of the writings of the early Church Fathers, from which virtually the entire New Testament can be reconstructed, scholars can and have detected the approximately 400,000 transcription errors that have crept into the textual body over the years. That sounds like a lot, but we're actually talking about what are formally referred to as
variants given that well-over 90% of these "errors" are actually misspellings, changes in word order (or syntax) word substitutions, omissions and the like, which do not change or distort the original meaning of the texts. These are unintended
variants for the most part, and the intended word substitutions merely go to the prevailing verbiage of the transcriber's native language relative to the language of the text being transcribed.
The other variants are intentional additions, typically of an explanatory nature. But that's not the end of the story as the overwhelming number of these were never intended to be transcribed as part of the original texts. Rather, they're explanatory notations or commentaries proffered by the transcriber regarding the historical contents of the original texts and this understanding simply got lost in subsequent transcriptions. Again, these are of no doctrinal or theological significance, but go to things like relative weights, measures, values of currency and the like.
Finally, there are the intended variants such as the apparent addition, in all probability, as that raised by you: Mark 16:9-20. Christians have been aware of this potentially gross variant for centuries as it is discussed by the early Church Fathers. Most of the earliest codices end at Mark 16:8, and the Fathers' consensus was that 9-20 were not part of the original text, despite the fact that it was included in many of the oldest surviving codices. Given that it has no significant doctrinal or theological impact, the historical solution of the Church since the latter part of the 20th Century especially has been to include it and a few others in Matthew and John with a caveat in translations:
Was Mark 16:9–20 Originally Part of Mark’s Gospel?
The takeaway here is that we know where these variants are in the textual body. Also we know their nature and how they got there.