In our shul, there are kohanim who I am sure are not observant and who even transgress in public. I am wondering if they are allowed to perform the priestly blessing for the community, or if their blessing might be a curse, G‑d forbid.
This question should rightly be addressed to, and answered by, the rabbi of your shul. I therefore only raise the issues involved, but defer to the rabbi of the shul regarding what should be done in practice in the specific instance of his community.
On the one hand, I empathize with your concern. It’s hard to imagine that hands which transgress the will of G‑d are fit to bless in the name of G‑d.
That being said, it is a positive Torah commandment incumbent upon kohanim to bless the Jewish People in the name of G‑d:
“Speak to Aaron and his sons, saying: So shall you bless the children of Israel, saying to them: ‘May the L‑rd bless you and watch over you. May the L‑rd cause His countenance to shine upon you and favor you. May the L‑rd raise His countenance toward you and grant you peace.’ The kohanim shall bestow My Name upon the children of Israel, so that I will bless them” (Nu. 6:23-27).
Based on this, Rambam writes (Tefilla 15:4), that even if a kohen transgresses major, severe sins (aside from murder and idolatry), so much so that the community is openly disdainful of him, the kohen nevertheless is commanded to bless. The reason for this, as explained by Rambam, is that we don’t say to a wicked person, “Increase your wickedness by declining to perform mitzvot.”
Anticipating your concern, Rambam continues, “Don’t wonder what worth is the blessing of such a sinner. The blessing is not dependent on the kohen but rather G‑d, as in the verse, ‘They shall bestow My Name upon the children of Israel, so that I will bless them’. The kohanim [even sinners] are to perform the mitzva incumbent upon them, and G‑d, in His great mercy, will bless Israel according to His will”.
One possible exception to this, which would bar a kohen from blessing, would be public desecration of the Sabbath, G‑d forbid. Since in many cases, brazen, intentional desecration of the Sabbath is considered tantamount to idolatry, it might also, like idolatry and murder, disqualify a kohen from fulfilling this mitzva. This opinion is accepted by the Mishna Berura (128:27, note 134).
However, this very question was presented regarding the community of South Africa to one of the greatest Torah leaders of recent times, Rabbi Chaim Ozer Grodzinski ztz”l (Vilna, 1863-1940).
Somewhat surprisingly, Rabbi Chaim Ozer answered that in this case, even kohanim who publicly transgress the Sabbath should be encouraged to bless the priestly blessing. His reasoning was that since these Jews were far-removed from observance, their Sabbath desecration should not be viewed as brazen and intentional, but rather without proper knowledge and understanding, as would be the case regarding a Jewish child who was taken from his family and raised among non-Jews. In such a case, innocent ignorance mitigates culpability.
Furthermore, he argued that if these kohanim were to be prevented from performing the priestly blessing, they and their children would forget their kohen status and thereby come to transgress in other ways, such as contracting marriages that are forbidden to kohanim, or exposing themselves to the impurity of dead bodies. In addition, he argued, enabling and encouraging them to fulfill their obligations as kohanim would maintain their connection to Judaism, to Jewish practice, and to their priestly pride – all of which might stem the tide of assimilation or even inspire a return to observance!