I'm generally annoyed by the "atheist who is converted" trope in TV & Films - but it's the writers' fictional character, so if they want to show Hank being converted over the course of the show, that's their business.
However, it seems wrong when they take a real life figure like Einstein who considered himself a "Freethinker" and did not believe in a god, nor in organized religion. You can't take one quote basically saying "the church did some good stuff to battle Nazis" (and there were plenty that collaborated!) and take that as supporting evidence that a non-believer Jew is secretly a 'new' apostle"? Give me a break. This is like those Mormons who post-humously convert people into their own religion.
Also, tiny nitpick - but why the heck didin't they just check the 1937 phone directory or the 1939 one? I'm betting that was still his phone number at those times. Even it if weren't the same, it was stupid of them not to think of it.