It is a well-known fact that, in most synagogues in the Diaspora, Kidush is recited at the conclusion of the Friday night Tefila, while in Israel few if any synagogues practice this tradition. As a matter of fact, the practice of reciting Kidush in the synagogue has always been accompanied with dispute.
In Masechet Pesachim (100b) we read that Rav was of the opinion that Kidush made in the Beit Knesset (synagogue) fulfills the halachic requirement for those present at the recitation of the Kidush. Shmuel disagrees and expresses the opinion that such a kidush is invalid since it does not meet the criteria of “Kidush Be’makom Seudah”. The Gemara asks: Why is it, according to Shmuel, that the custom is to make Kidush in the Beit Knesset? The answer suggested is that the Kidush is meant for the guests who reside and eat at the Beit Knesset. For those who eat at the Beit Knesset it is the place of their meal (seudah) and therefore the Kidush recited in the Beit Knesset fulfills the criteria for a proper Kidush.
Two questions are raised by the commentaries in reference to this answer. The first relates to the fact that people ate and slept in the Beit Knesset, which stands in contradiction to other sources that prohibit such use of a Beit Knesset. The other question raised is: What is the Halacha when no guests eat at the Beit Knesset We will deal only with the later question, leaving the issue of proper use of a Beit Knesset for another time.
Though, usually in a dispute between Rav and Shmuel we rule like Rav in matters of fulfillment of Mitzvot, in this case since several other Amoraim agree with Shmuel that kidush must be made in conjunction with a meal, they would therefore also agree that the Kidush made in the Beit Knesset is invalid. Therefore, it is the opinion of the poskim to accept Shmuel’s opinion, in this case, over that of Rav. That being the case, we must understand what is the basis for reciting Kidush today in Batei Knesset of the Diaspora that are not used by guests. This precisely is the reason that in Israel the custom is not to recite Kidush during the Tfila.
Several answers can be found in the commentaries to this question. Rav Netrunai Gaon suggests that the Kidush made in Beit Knesset is for health purposes since the wine of Kidush is supposed to be a preventive measure for eye ailments. The primary problem with this answer is that in the Gemara we find no mention of such a reason for reciting Kidush in Batei Knesset.(See Tur OC 269)
Another possible explanation is found in the Rosh in the name of Rabenu Yona. Rabenu Yona claims that since wine is only a Rabbinic requirement, the Kidush in Beit Knesset serves those who may not have wine to recite Kidush at home or for those who would not recite the Kidush at home for other reasons. This explanation is dependent on the assumtion that wine is only of Rabbinic origin and that the Biblical (Deorita) requirement is not fulfilled by reciting the Shmone Esre. (See Aruch Hashulchan OC 269/3)
A third approach can be found in the Ran in Pesachim. The Ran preposes that initially Kidush was recited in Batei Knesset for the guests who ate there. Later even though guests were no longer housed in Batei Knesset the custom remained intact as we find in other similar situations. One such example presented by the Ran is the recitation of the Birkat Me’ein Sheva (otherwise known as Magen Avot) after the silent Shmone Esre Friday night. The Bach rejects this understanding, claiming that while Birkat Me’ein Sheva is found as a formal instituted Bracha in the Gemara, Kidush in Beit Knesset is only a custom. The Aruch Hashulchan argues that from the wording of the Gemara in Pesachim it would appear that Kidush in Beit Knesset was formally instituted.
We may propose an understanding in the Ran, which would avoid the problem the Bach raised. The Ran differentiates between the motivation of the custom to recite Kidush in Beit Knesset and the form it took when implemented. The original motivation was as the Gemara explains for the need of those who ate and dwelled in the Beit Knesset. Since the need was recognized, the Kidush was instituted as part of the Friday night Tefila. Even when there was no need to recite the Kidush for guests, the practice remained as part of the format of the Tefila. This is precisely what occurred with Bracha Me’ein Sheva. It’s origin was to delay the conclusion of the Friday night Tefilah, but even after this delay was no longer needed it remained as part of the Tefila.(See Ran Pessachim to daf 100b and Rashi Shabat 24b)
This understanding clarifies several points in the Gemara Pesachim. First, why was the Kidush recited in the presence of the entire community instead of supplying wine to the guests who could then recite Kidush before eating. Second, why was the Kidush recited by a member of the community and not by one of the guests. Our explanation also clarifies why the Kidush is recited before Aleinu. Since it is meant to be a public declaration of the sanctity of Shabat as part of the Tefila, the Kidush is recited before the closing Tefila while the Tzibur is still gathered for Tefila.
Since the Kidush recited in Beit Knesset is not meant to fulfill the Halachic requirement of Kidush, the wine should be given to children to drink. If no children are present then an adult should drink the wine making sure to drink more then a Reviit depending on the opinion that a reviit of wine constitutes a seuda for the purpose of Kidush.