The Shulchan Aruch states, based on the concluding passage in Masechet Rosh Hashanah, that Birkat Kohanim recited in a shul is ineffective with respect to people standing behind the Kohanim. In introducing the Mitzvah of Birkat Kohanim, the Torah Hakedosha in Sefer Bamidbar 6:23 states: “Amor Lahem – Say to them.” This implies that the beracha must be recited in the manner in which people speak to one another.
Since one generally faces the person to whom he speaks, the Kohanim must likewise face the congregation when they declare this berachah. Hence, those standing behind the Kohanim must move in front of the Kohanim to ensure that they are included in the berachah.
The Shulchan Aruch emphasizes the point stating that those standing in front of the Kohanim are included in the berachah even if an “iron barrier” separates them from the Kohanim. As long as they are positioned in front of the Kohanim, they are included in the berachah regardless of any object that stands between them.
In addition, the Shulchan Aruch rules that those directly to the side of the Kohanim are considered to be in front of the Kohanim for the purposes of this Halachah. Ha’Rav David Yosef (a”h) in his sefer Halacha Berurah adds that those standing directly to the sides of the Kohanim should turn their faces towards the Kohanim during the berachah, so that they will be facing them as required. However, he noted, people situated to the side and in front of the Kohanim, on a diagonal, need not turn their faces towards the Kohanim.
The Shulchan Aruch adds that those who cannot come in front of the Kohanim due to extenuating circumstances, such as illness or that their frailty does not allow them to reposition themselves, are nevertheless included in the berachah. Since it is due to circumstances beyond their control that they cannot come in front of the Kohanim, the beracha applies to them even though they stand behind the Kohanim.
The same applies to those who cannot come to shul due to extenuating circumstances. They too are included in the Kohanim’s berachah even though they cannot attend services. The Kohanim must therefore have in mind when reciting the berachah that it applies not only to those who stand in front of them but also to all those who sincerely wish to come to shul but are unable to do so.
Clearly, however, those who do not come to shul due to laziness or for their own convenience are not included in the berachah.