Judaism is the original and most ancient monotheistic religion with the Holy Torah as its foundation. The RAMBAM’S 13 Principles of Faith best describe the foundation of the Jewish faith:
G-d is the Creator and Guide of everything that has been created. He alone is the cause of all that exists.
G‑d is absolute and unparalleled unity, He is one.
G-d is incorporeal, He is free from all anthropomorphic properties and He has no likeness at all.
G-d is eternal, the first and the last.
G‑d should be worshiped exclusively. He is the only one to whom it is proper to pray to. One should not pray to any foreign false gods.
All the words of the Prophets in the Torah are true.
7. The prophecy of Moshe Rabbeinu was true and his prophecy is superior to all prophets who preceded him and all those who followed him.
8. The whole Torah (Written & Oral) was given to Moshe Rabbeinu and it is the same Torah we possess today.
9. The Torah will not be changed, and that there will be no another Torah given by G-d.
10. The belief in G‑d's omniscience and providence.
11. G-d rewards those who observe His commandments and punishes those who transgress His commandments.
12. The coming of the Mashiach and the messianic era.
13. The resurrection of the dead.
What makes Judaism different from all other religions? It is the only religion that started in front of millions of witnesses, when G-d gave Moshe Rabbeinu and Am Yisrael the Holy Torah on Mount Sinai. While some religions are based on action without belief, belief without action or simply birthright, the Torah teaches us that all three (Birthright, Belief and Action) parts are what makes a Jew Jewish. Although one who converts to Judaism does not have the biological birthright of a natural born Jew, his/her actions of believing and following the Torah completely leads to them being gifted a new NeShama (soul) from Heaven that can be at a higher level than one of a natural born Jew. Similarly, a Jew that has not followed a life of Torah, and thereby putting their spiritual status of Judaism on suspension, can acquire it back and even get his NeShama to a higher level than nearly anyone by doing the holy act of TeShuva (returning to the ways of HaShem and His Torah).