Judaism does not teach that "eternal rest" is guaranteed only to those who practice Judaism.
For instance, when asked if Jews think their religion is the right one, the answer is usually: "For whom?"
Unlike what many of us may be familiar with due to our exposure to Jehovah's Witnesses, Judaism does not agree that religion has an exclusive on truth. At the same time, it neither promotes others to join Judaism nor any religion for that matter. It also promotes no search for ultimate truth or gives "truth" of any type a salvific quality.
Jews see their cultural response to their concept of God as "their religion," but at the same time do not teach that it is the way for others who are not of Jewish origin.
Spiritual and secular paths other than Judaism are fully valid and can be beneficial as long as they conform to the seven basic laws of all humanity, often referred to as the seven Noahide Laws: do not serve idols, do not curse the God of Abraham, do not murder, do not pervert sexuality or blatantly live outside the sexual mores of society, do not steal, live justly, and treat all animals mercifully.
One need not worship the Jewish God or even acknowledge God, so religion is not necessary for those outside of a covenant relationship with the Creator. If there are ultimate truths or axioms, discovering and holding these guarantees no salvation of any type.
If there is a World to Come in which an eternity of bliss awaits, it is open to all who are just, not just Jews. The idea of "truth," however is therefore separate and, again, not a requisite. If there is truth anywhere, it might be found in some religions as well as outside of religion in general. But knowing the truth about anything is very different from being honest, honorable, and just.
