The Jewish idea is that the Torah of Moses is a truth for all humanity, whether Jewish or not. The Torah (as explained in the Talmud - Sanhedrin 58b) presents seven mitzvot for non-Jews to observe. These seven laws are the pillars of human civilization, and are named the "Seven Laws of Noah," since all humans are descended from Noah. They are:
- Do not murder.
- Do not steal.
- Do not worship false gods.
- Do not be sexually immoral.
- Do not eat a limb removed from a live animal.
- Do not curse God.
- Set up courts and bring offenders to justice.
Maimonides explains that any human being who faithfully observes these laws earns a proper place in heaven. So you see, the Torah is for all humanity, no conversion necessary.
As well, when King Solomon built the Holy Temple in Jerusalem, he specifically asked God to heed the prayer of non-Jews who come to the Temple (1-Kings 8:41-43). The Temple was the universal center of spirituality, which the prophet Isaiah referred to as a "house for all nations." The service in the Holy Temple during the week of Sukkot featured a total of 70 bull offerings, corresponding to each of the 70 nations of the world. In fact, the Talmud says if the Romans would have realized how much they were benefiting from the Temple, they never would have destroyed it!
Today, there are many active groups of non-Jews called "B'nai Noach" who faithfully observe the Seven Laws of Noah.
There is an excellent book on the topic:
"The Path of the Righteous Gentile"
by Chaim Clorfene and Yakov Rogalsky
"The Real Messiah: A Jewish Response to Missionaries"
by Aryeh Kaplan