Exactly,
I always question when someone says "Conversion" from Christianity? is there such a thing? I can imagine you might decide to become an "Atheist" which is a lack of moral strength IMO..
But since you can be a Jew or Gentile Christian, I do not see what Kerry's brother converted too? if he became an Atheist then I guess he had a immoral conversion, if he is part of the Jewish faith he can still be a Christian as the Apostles and the early christians (All Jews) were...up until Antioch...
Well, its pretty straightforward. He converted to Judaism. A Reform conversion, from the sounds of it, although that isn't clearly stated in the article.
Just because this concept doesn't fit into your worldview doesn't mean that people don't do it all the time.
if he is part of the Jewish faith he can still be a Christian
He may never have been a believing Christian. Or, if he once were, he rejected those beliefs somewhere along the way. At any rate, when he converted to Judaism, he no longer held specifically Christian beliefs such as the trinity, the incarnation, etc.