Q. Would a kohen who leaves Judaism and later returns be allowed to act as a kohen and duchan (pronounce the priestly blessing upon the congregation)?
A. This question was put to Rashi in the 11th century, at a time when the Crusades had led to problems of apostasy. The issue was whether a person who left Judaism was to be considered a Jew or a gentile in the eyes of Jewish law, with the associated question of the former apostate’s status when he came back to Judaism. Rashi’s responsa on these and other subjects were gathered in 1943 by Israel Elfenbein.
Responsum no. 170 in Elfenbein’s collection rules that a kohen who has returned to Judaism may indeed duchan.
The analogy is given of a kohen who has a physical blemish; the Talmud says that he may not minister at the altar but may bless the people, unless the blemish is on his hands, which he raises to pronounce the blessing (Ta’anit 27a; Megillah 24b). The kohen who was an apostate has a blemish on his record, but if he is now a believing and practising Jew he is permitted to utter the priestly blessing. Rashi adds that this applies all the more nowadays when there is no Temple and no sanctuary.
Rabbenu Gershom comes to the same conclusion and warns that people should not embarrass the kohen by saying to him, “Remember your previous situation”. This is not only “verbal oppression” but will also discourage people from repenting and returning.