Answer

Although the sin of the מרגלים themselves was much more severe than that of the rest of their generation, they too ___ have a part in עולם הבא, because they too were present at מתן תורה, when all בני ישראל entered a ברית, a covenant with Hashem.

Just as a ברית between two friends ensures that no matter what happens in the future they will remain loyal and connected to each other, our ברית with Hashem is eternal. And even if mistakes are made along the way, it does not take away that eternal connection.