How Jewish Proselytism Really Died
It would seem that the restrictions on Jewish proselytism date not from the era in which the Roman Empire became a Christian one, but from the period of the Jewish Revolts which started in AD 66 and culminated with the crushing of Simeon Bar Kochba in AD 136. Whatever one may think about Rome's policies from a religious angle, from a purely strategic point of view, the restriction on Jewish proselytism would certainly have made a great deal of sense, considering how many times it had been necessary in the space of 70 years to crush Jewish rebellions: why permit such a troublesome people to recruit yet more followers to their ranks?
Being the staunch secularist that I am, I can't help but seeing the failure of the Jewish revolts, and the tragic consequences that followed from them, as an abject lesson in the dangers of allowing fanatical religious mumbo-jumbo to blind one to geopolitical realities. There simply was no way these rebels could have hoped to have won against a Roman Empire at its prime, possessed of the most sophisticated military organization the world had yet seen, and one that is still looked upon with admiration even by military professionals today; any rebellion against such an authority could only have had one possible result, as for Rome to have permitted one small gang of rebels in Judea to have their way would have been to give the green light to rebellion throughout the extent of the empire.
One hopes today's Israeli leaders will have the good sense to take the appropriate lessons from that sad period in Jewish history and strive to come to some accommodation with their neighbors, however disagreeable some of these neighbors may be, rather than allow themselves to be spurred on to self-destruction by pie-in-the-sky "prophecies" of a "Third Temple" or a "Greater Israel" in a bunch of old religious texts. Messianic nonsense is even more of a danger to Israelis than it is to those of us who must live side-by-side with "rapture" obsessed fundamentalists.
PS: The legislation forbidding gentile conversion to Judaism was passed by the Emperor Antoninus Pius, who made it a capital crime for Jews to circumcise non-Jews - see this article for more information.
PPS: A slight point of clarification - although many sources have it that it was the Emperor Hadrian who forbade circumcision throughout the Empire only for Pius Antoninus to issue limited permission for Jews alone to practice the act on their sons, there is actually no good evidence to indicate that Hadrian ever initiated such a law. What does seem to have occurred is that Hadrian issued a rescript banning castration*, and that somehow word of this reached the residents of Judea in garbled form as an edict against circumcision, initiating the futile revolt in which Simeon Bar Kochba and Rabbi Akiva were to play key roles.
*A practice I hope we can all agree was eminently deserving of complete prohibition.