Q. How can a rabbi who is not a kohen be allowed to recite the priestly blessing for a Bar- or Bat-Mitzvah or a bride and groom?A. The priestly blessing pronounced during the Amidah cannot be given by a non-kohen. The Torah says, “Thus shall you (the kohanim) bless the Children of Israel” (Num. 6:23; see Ket. 24b).
The kohen raises his hands in a set way during the blessing. A non-kohen may not raise his hands in the set way as part of this ceremony, though Rabbi Yose, a non-kohen, said that if his colleagues invited him he would not hesitate to participate (Shabbat 118b).
Some say that non-kohanim were excluded only in Temple times; the halachic consensus is that the exclusion continues to apply (Shulchan Aruch, OH 128:1), but this is in the context of the official duchaning ceremony during the statutory services.
However, citing the b’rachah when blessing children on Friday night or blessing a Bar- or Bat-Mitzvah or a bride and groom does not transgress the law. To avoid any misunderstanding, some rabbis use introductory words such as “May HaShem bestow upon you the threefold Scriptural blessing…”
Unless he is a kohen, the rabbi should not raise his hands in the set kohanic way. Some raise one or both hands in a non-statutory fashion, but using only one hand for this purpose has the drawback of resembling a church custom.