Were the founders of Christianity Jews or not?

"On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country however persons engaged in agriculture may freely and lawfully continue their pursuits because it often happens that another day is not suitable for grain-sowing or vine planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost."

Thus decreed Constantine the Great in 321, marking the beginning of the 2-day weekend. It was only later that resting on the Sabbath was prohibited, thus making the practice of Messianic Judaism illegal.

Since the idea of religious freedom and a secular state was first proven in America, it has become possible--if not always practical--for Jews and Christians alike to observe the first day of the weekend as a day of rest.

At hopeabbey.com [no longer available] Jake has a very provoking blog post on the deprecation of Jewish Christianity, in which he asks the question,
"Is there any other religion that paints its first adherents as people who never “got it,” never understood their own religion?"