This passage from the Tosefta is concerned with the rabbinic prohibition against reading by the light of an oil lamp on Shabbat. This law comes to prevent the reader from tilting the lamp and thereby increasing its flame, which is prohibited on Shabbat. Rabbi Ishmael, who was active in the second century CE (before and after the Bar Kokhba revolt), describes a scenario when he started reading on a Shabbat night, and he almost tilted the lamp. In this situation, he came to appreciate the significance of this prohibition. However, according to Rabbi Nathan, Rabbi Ishmael indeed tilted the lamp, as it is recorded in Rabbi Ishmael's notebook: “Rabbi Ishmael the son of Elisha tilted the oil lamp on Shabbat. When the Temple is rebuilt, he will bring a sin-offering.” Interestingly, even though this passage does not explicitly make assertions regarding the rebuilding of the Temple or its timing, this story affirms that a next Temple is plausible and it could be understood to anticipate it in the near future. A parallel to this story is transmitted as a baraita (tannaitic citation) in the Jerusalem Talmud Shabbat 1:3, 3b:

תני. שמא ישכח ויטה. אמ' ר' ישמעאל. אני אקרא ולא אטה. ושכח והיה קרוב להטות. ואמ'. גדולים הן דברי חכמ' שאמרו. שמא ישכח ויטה. ר' נתן או'. היטה אותו ממש. וכתב על פינקסו ואמ'. ישמעאל בן אלישע היטה הנר בשבת. לכשיבנה הבית יהא חייב חטאת.

It was taught in a tannaitic tradition: Lest he forget and tilt [the lamp]. Rabbi Ishmael said: “I will read and not tilt [the lamp].” But he forgot and almost tilted [it], and [then] he said: “Great are the words of the sages who say ‘Lest he forget and tilt.’” Rabbi Nathan says: “He actually tilted it and he wrote in his notebook (or on his writing tablet) as follows: ‘Ishmael the son of Elisha tilted the lamp on Shabbat. When the Temple is rebuilt, he will be liable for a sin-offering.’”

These parallels are very similar (see also Babylonian TalmudShabbat12b). However, whereas the Jerusalem Talmud notes that: “he will be liable (ḥayyav) for a sin-offering” when the Temple  is rebuilt, the Tosefta states: “When the Temple is rebuilt, he will bring a sin-offering.” Through this subtle difference in language, the Tosefta’s statement seems to be more confident of an impending rebuilding, since it is less conditional. In any case, even though this text from the Tosefta is not explicit in this projection, it may be grouped with several other tannaitic texts (mostly from the Tosefta), that articulate the proximate rebuilding with certainty or, at least, a notion that such a rebuilding is feasible.