I like what "Add A Tag" said, and agree with him/her. I would add that our biographies of Alexander the Great were all written centuries after his life and death. Our biographies and the history of Rome we have were written 75-100 years after the events. These records are secondhand/thirdhand or even further-down-the-line hand accounts, and yet we regard them as history.

Even in the news we read on the Internet or hear on TV every night is often secondhand or thirdhand information. They often say, "Our sources claim that..." Are most news stories to be discarded then as unreliable?

I think it's quite possible for me to hear someone give an account of a movie they saw or something they heard on the news and for me to pass it on reliably. We're not that incompetent, and if we are, all disciplines are in trouble.

In like manner with his/her post, I think there's good reason to believe Matthew and John are firsthand accounts, Mark is a secondhand account (from Peter), and Luke's is a well-researched and reliable account. I'd be glad to discuss it further with you.

You are proposing a principle, and a case to evaluate with that principle.

The Principle:

A secondhand/thirdhand account is not reliable.

The Case:

The Gospels are (or may be) secondhand/thirdhand accounts.

By the principle, they are not reliable.

Now, I would deny both premises (two of the Gospels are firsthand, and I don't hold the principle you use). But, let's explore the consistency of the principle as well. Here's another case:

Every US History textbook is a secondhand/thirdhand account.

By the principle, we can't trust any history textbook.

The principle is one of ultimate skeptical rejection that is not applied anywhere else, and if you want to apply it to the Gospel accounts, you also have to apply it to other accounts. Then you have to ask the question: how do you know that the firsthand account actually is firsthand? From the account itself? Or did you hear that secondhand?

Basically, you run into a lot of problems trying to actually use this principle as a reason to reject the validity/reliability of some accounts, because in the end you must then reject all accounts.

How reliable can the gospels be if they are thirdhand accounts rather than firsthand or even secondhand accounts?