Early Christians weren't called Christians; they were called Jewish Schismatics, meaning that they were still considered Jews but just a bit weird, much like the Hassadic vs Orthodox vs Conservative vs Reformed today. This started at Antioch in the 4th century, not in the first century and popularly conceived.
Constantine the Great (274 - 337) was the ruler of the known world, born in Rome an died in Byzantine, established Christianity, now known as Eastern or Greek Orthodox, as THE official religion of the empire. He saw a vision of the Chi/Rho and the words "En Touto Nika" or in Latin, "in hoc signo vinces" which comes out in English as "In this sign Win". So he had his troops adorn their shields with the Chi/Rho and Cross - and they won. For more information on him, his enforcement of Christianity and toleration/destruction of the Roman gods in mostly at http://en.wikipedia.org/wiki/Constantine_the_Great_and_Christianity#Conversion.
Now, here's a thought: What if we consider Christianity NOT a religion but a political movement and the exultation of Y'Shua (Jesus) as a divine manifestation of G-d himself. Well, the problem there is that this goes totally against the Torah, Talmud, TaNaKh and all other Jewish writings - he can't be G-d and still be Jew. There is only one G-d and, in my own opinion, his name is not Allah. However, I have been informed that Allah should be translated into English as "The G-d" meaning the same G-d as the Jews. Neither is it Jesus, a bad translation of Y'Shua that comes down from the Latin and Greek, to German, to English. You can check out more on Antioch at http://en.wikipedia.org/wiki/Antioch.
So, then, can a Christian become a Jew? Or is he/she already a Jew. So long at they proclaim Y'Shua as G-d, then no. But, if they accept Y'Shua a great Rabbi whose pronouncements were mis-quoted in only a couple of places, probably so. Then the New Testament becomes not holy scripture as is the Masoretic text, but further explanations of the early Christian movement that later conquered the world.
Now we have Islam to cover in another blog. I'll be nice...
SDG
Yaakov