A Historical Question

In First Things this month, Richard John Neuhaus starts of his The Public Square column with a piece on David Klinghoffer's book Why the Jews Rejected Jesus. Mr Neuhaus has little good to offer about Mr Klinghofer's book, but does make the following observation. He states:

Scholars generally agree that in the first century there were approximately six million Jews in the Roman Empire ... About one million were in Palestine ... Some scholars have noted that by the fourth or fifth century, there were only a few hundred thousand, at most a million people who identified themselves as Jews. What happened to the millions of others? The most likely answer it is suggested, is that they became Christians. What if the great majority of Jews did not reject Jesus?

Unfortunately these statistics are unattributed, which to be honest is not surprising in a book review or opinion piece. My question is, where would one look to verify these figures? Does anyone reading this have any suggestions?

Thanks.