It seems that this question is at the crux of the issue people have about various forms of Judaism. Discussions of denominations are against the rules of this forum because it sidetracks us from our purpose: to educate Jews.
Judaism is simply Judaism. Anything that "spins away" from Torah whether it calls itself Karaism, or Sadduccee or any other name is NOT Judaism. The "tag" Orthodox Judaism is a fairly new one given to Judaism itself. Judaism has not changed -- which brings me back to the topic at hand -- what does it MEAN to be a Jew?
First let's start with "who is a Jew?" A Jew is a person born to a halachically (Jewish law) Jewish mother. So far this has nothing to do with following the religion of Judaism.
One may become a Jew by agreeing to take on the covenant that Jews made with G-d at Har Sinai (Mount Sinai). This person must observe the mitzvot and a court of Jewish law (called a Beit Din) must agree that they are living as a Jew. The final action is for this person to submerge in a mikvah (a ritual bath) and the emerge as Jewish as one born a Jew.
Jews made a promise to G-d at Sinai. G-d made a promise to us there as well. The ten utterances where G-d Himself spoke to the entire nation including those not yet born sealed this pact.
Therefore to live as a Jew is more than to be a monotheist. It means that as we reach maturity we are bound to the covenant agreed to over three thousand years ago at Sinai. We can not pick and choose which mitzvot to observe (today I'll ignore the laws of kashrut, tomorrow I'll make a fire on Shabbat. . .). We must observe all of those that pertain to us the best that we can.
The problem with various spin-offs of Judaism is that a group of people get together and decide to not only cahnge the mitzvot but to do so as a "standard" for others. They set themselves up as an authority figure for others and put in place a different set of rules (the Karaites wound up with their OWN version of an oral tradition), and the poor children raised in this environment simply don't know what they don't know.
Every Jew is a precious soul to the nation of Israel (not the modern country, but the PEOPLE Israel). No one should judge a fellow Jew or look down upon any of them. The problem lies not with people, but with what they have been led to believe. Parents are responsible for teaching children (l'dor v'dor -- from generation to generation) and one cannot blame the child if they are not taught.
Now, the question arises, if one does not believe in G-d (and is Jewish) or if one follows a heretical path is one an apostate and thus kareit (cut off from G-d)? Or is this a situation of tinok shenishba?
Tinok shenishba is where a person has been raised without a proper understanding and education of G-d. They are like a child who was kidnapped as a youth and never taught Torah and their responsibilities. A tinok shenishba never had the proper guidance and thus doesn't recognize their responsibilities to lead a Jewish life, following the mitzvot, he or she is "not held accountable for not living in accordance with the Torah until such time as they do become aware of their responsibilities. This is the very purpose for Jewish outreach.
Throughout the thousands of years that Judaism has existed there have been Jews who have assimilated into other religions -- often out of a desire to be part of the mainstream. This was true in Spain (Spanish Inquisition), it was true in Germany (the "englightenment") and it is true in America.
But the path of the Jew has never changed.












