The professor is only correct if he defines a weak message as one which does not garner much support.
That would however make the person who claims that christianity is a strong message correct.
However if he was saying loads of supporters so must be strong in truth terms that is incorrect or conversely if few supporters then the message is weak in truth terms then that is also incorrect.
Philosophicalsociety.com gives this as an example. It is close to what the professor is arguing imho
''argumentum ad populum -- This fallacy occurs when an argument panders to popular passion or sentiment. When, for instance, a politician exclaims in a debate that his opponent "is out of step with the beliefs of everyone in the audience," he/she is committing the fallacy. The legitimacy of a statement depends not on its popularity, but on its truth credentials.''
In which Lieutenant Pigeon makes several errors:
1. Weak vs strong is ambiguous because it implies some relationship to the truth value of the statement – the sun orbiting the earth would have been a “strong” message once using this term. Persuasive vs not persuasive is closer to it, and the point made is that Jesus does not appear to have been persuasive for those who heard him.
2. “Christianity” (or actually the bewildering varieties of it) is only persuasive for those who identify as Christians, and the extent to which they are persuaded rather than got at before their critical faculties have developed is moot to say the least.
3. An
argumentum ad populum is the assertion that popularity implies truthfulness. The Prof said no such thing – rather he merely made the point that the Christ narrative only caught the wind with the distance of space and time – ie, with the opportunity for the stories to be added to, embellished, edited for their eventual audience.
4. If the quote is correct, then it’s wrong – or at least incomplete. When a “politician exclaims in a debate that his opponent "is out of step with the beliefs of everyone in the audience," he/she is
not committing the
ad pop fallacy. This could just be a statement of fact. For there to be an
ad pop the politician would also have to say, “because there are more people in the audience who think X and only one of you who thinks Y, the audience must be correct.”
And again none of that reflects what the Prof actually said in any case.
Apart from that though…