Surely you are familiar with the usual pattern in theistic religion:
"In the past, God gave a special man information about Himself. Ever since then, everyone who wants to know about God has to depend on that man and his followers."
So nowadays, everyone who is not that special man to whom God revealed Himself, is in the group of those who have to depend on that special man.
"All the information that humans can possibly have about God necessarily comes via other people" is true for all those people who don't have first-hand information and who have to depend on that special man (and his followers).
Which, nowadays, in some religions, means that everyone has only such second-hand or third-hand information about God.
And even in those religions that teach that one only needs to "look within to find God": even in such an instance, one would still be trusting others that that which one finds within, is indeed about God; one still wouldn't have first-hand knowledge of God.
Unless that usual pattern is actually historically accurate, using it is not a valid counter-argument since it assumes the truth of that assertion and draws conclusions countering those achieve by simple logic. Parsimony and substantiation both favor the latter.