The Torah (Devarim 16:5-6) teaches that one cannot bring the korban Pesah “in any one of your gates” – that is to say, in one of the communities outside of the Temple; rather it must be sacrificed in the place chosen by God. This passage is understood by the Sages to teach a number of halakhot connected with the sacrifice.
On its simplest level, that pasuk teaches that the korban Pesah must be brought in the Temple. Rabbi Shimon understands this to mean that someone who brings the sacrifice on a bamat yahid – a private altar – will be held liable for transgressing a negative commandment. This only holds true, however, when private altars are forbidden, when the Jews all “enter through the same gate,” i.e. when the Temple is standing. During a time when private altars are permitted, the korban Pesah can be brought as a private sacrifice.
Prior to the erection of the Temple, there were times when an individual was allowed to build a bamat yahid where he could bring sacrifices, even if he was not a kohen. Generally speaking, the sacrifices that were brought on a bamat yahid were voluntary ones (olot and shelamim); communal sacrifices were brought only on the bama gedola – the public altar in Nov or Giv’on. Once the Temple was built, all private altars became forbidden, although a perusal of the stories in Sefer Melakhim makes it clear that people continued bringing sacrifices to God on bamot yahid, activities fought by the prophets.
Rabbi Yehuda teaches that the pasuk can be understood to mean that you cannot bring the korban Pesah “in any one” (leaving off the last clause of the passage). According to this reading, a korban Pesah must be brought as a group effort; it cannot be brought by an individual, even if that individual could eat the sacrifice on his own. Rabbenu Yehonatan explains that Rabbi Yehuda believes that the word ehad – one – is extra, as the Torah could have said that you cannot sacrifice the korban Pesah within your gates, without saying “any one of your gates.” Therefore he understands that the pasuk is teaching us to broaden the context of the korban Pesah to a group setting.