How did the Jewish god become an International Sensation?
I would guess through Roman rule, Jews were able to travel all over the Roman Empire. There were Jews in Rome already when Christians arrived there, and there were Jews in many other parts of the Empire. These Jews could have been slaves of Roman traders or wealthy merchants themselves, moving animals, food, and valuables from Egypt and Greece to Rome and other cities [including, of course, Jerusalem]. For some reason - I do not yet know why - some Romans were interested in Judaism but did not become exclusive followers of the Jewish G-d. These Romans attended synagoge [but were in the back] and received Jewish teaching but did not abandon the Roman way of life [adopting a Jewish diet, observing the Sabbath, and polytheism].
Christians, of course, widely popularized the Jewish G-d, however mistaken an understanding that they had of Him in the eyes of ancient and modern Jews.