Why did the People of the Cave equate stoning with being returned to their disbelieving creed, in that both lead to never achieving salvation, despite the fact that stoning for the sake of Islam might lead to salvation in the Hereafter?

Allah commands reflection on the Quran, and Surah (definition) Al-Kahf in particular. Verses 19-20 of Surah Al-Kahf speak about the awakening of the People of the Cave after a long sleep and their questioning about the duration of their stay. Then, they decide to send one of them to buy good food from the city, with extreme gentleness and without attracting attention, fearing the people of the city who would stone them to death or force them back into disbelief. In this case, "and then you will never succeed" refers to the failure of anyone who returns to the religion of disbelief, meaning loss in both this world and the Hereafter.