What do Jews believe in? It's a question that every Jew gets asked at regular intervals because Christians are generally confused by this religion which seems to believe in so little, and most Jews are a little confused too.
Here's my personal answer: There's a guy who does a lot of things, he's everywhere, he gave us a book, do what the book says.
There isn't much more than that in Judaism, and who cares?
I've long harbored a personal theory about that some would find controversial if ever printed; here goes. Faith, or emunah, as it's understood today, is tacked onto Judaism by goyisher thinking; a competely inorganic fit with a religion which prizes ethical conduct much more than good intentions. The Tanakh asks much more than the New Testament in laws to obey; but the New Testament tacks wall to wall with verses about faith's importance, while the Tanakh is far more modest in its demands on our beliefs. Avraham and Moshe constantly question the veracity of Hashem's promises. Yaakov was of course given the name Israel, which literally means 'he who wrestles with God.' The Torah doesn't implore us to have faith, just to trust in Hashem's covenant.
Judaism without doubt is less Judaism than Judaism without faith. Faith is only required in a theology with values formed in a period when beliefs are proven false. In the Abrahamic era, there was no Greek scientific tradition to contradict. Judaism was, as best Avraham knew, the most up-to-date knowledge of 1000 BCE. Christianity rejected intellectual inquiry's findings, and spent the next millennium and a half reconciling its beliefs to the knowledge whose progress it halted. Jesus was a believer; at Christianity's founding, Jesus was a practicing Jew, one of an untold number of sages whose followers thought him Moshiach. Avraham was a skeptic who founded Judaism by rejecting the faith of his fathers.
Perhaps therefore, doubt, not faith, is Judaism's root. For at least 3000 years, Judaism survived the astronomical odds that our peoplehood would disappear in every century. We survived not because our beliefs are strong, but because they're weak, and our weakness is our greatest strength. The Torah has not a single law for which we haven't created an entire literature of exceptions and injunctions surrounding it, because long experience teaches Jews that the inability to adapt to every situation is what gets people killed. Why have a Rabbinic tradition that interprets every law for its remotest circumstances? So that you have instructions for every situation and precedents to consult, and therefore live with something more solid than faith that Hashem will provide without human help.
In the family tree of every secularist with Jewish genes, there's a moment when somebody had to leave the Jewish worldview behind to become secular. I guarantee that act was a trauma for everybody: the person who left, their parents, maybe their siblings, and eventually their children. Judaism does many things well, but respect for those you disagree with is deliberately not one of them.
Judaism depends on every community member as a check and balance on everybody else - you critique and interpret everyone's actions every minute, you learn from early childhood onward to treat those you hate with understanding and those you love with skepticism. A person who rejects our checks and balances for Christianity or (zol Gotter Pitten...) intermarriage, hits most Jews at a level too primal for the American worldview to understand - I'm not even sure I understand it.
Some Jews live entire lives in gentile communities and go to shul thrice a year; but froth at the mouth when a younger cousin they see thrice a decade marries a gentile. Any Jew with half-Jewish grandchildren probably knows the uproar against intermarrying Jews is dangerously extreme, so allow me to make yet another provocation: perhaps intermarriage feels like a betrayal because Judaism, properly practiced, is already so close at root to agnosticism. The purpose of Judaism was never what we believe but what we do. Perhaps before we're a faith, we're a covenant of ethics cast adrift in a sea of faiths, forced therefore to become a faith against the wishes of He who bound us to His Covenant.
Why is this human history first era when the probability of genocide against Jews is less than 50%? Is it because of the State of Israel? Or is it because of the mentality that allowed the State of Israel to exist? Modern liberal skepticism looked at the Jewish mentality and realized we'd been there all along, and that's why modernity let us prosper.