No, he's saying that all untestable statements that were and are made on the same basis as those statements that were untestable at the time they were made, but which later turned out to be false, can be infered to be untrue as well.
Because we have evidence that this method of understanding the world - religious inspiration - is inaccurate.
The evidence given for god is the testimony of those who say that god has communicated with them. We have reason to believe that this testimony is false as applies to all the predictions it made that we have been able to test.
This suggests that the methodology of religious inspiration is not useful.
Note he doesn't claim that this proves God or Christianity wrong. It just suggests that we should reject it provisionally - until new evidence comes to light. The same point applies to psychics.