For so long as the Jew has even one ally, he will be convinced - in his smallness of mind - that his salvation came from that ally. It is only when he is alone - against all of his own efforts and frantic attempts - that he will, through no choice, be compelled to turn to G-d.
What convinces is conviction. Believe in the argument you're advancing. If you don't you're as good as dead. The other person will sense that something isn't there, and no chain of reasoning, no matter how logical or elegant or brilliant, will win your case for you.

