(ז כ) “אלקיכם ‘ה אני כי קדשים והייתם והתקדשתם”
In Maseches Brachos (53b, see Maharsha Vol. I, ibid) Chazal explain that this passuk relates to Birchas Hamazon. Harav Naftali Katz, author of Semichas Chachamim, wrote: “If the mevarech is called kadosh how much more so is the one who answers amen called kadosh, because ‘the one who answers amen is greater than the mevarech.’” (Brachos ibid)
The Pele Yoetz wrote that the Torah placed the passuk (Bamidbar 5:10): “V’ish es kedoshav lo yihiyu” next to the parashah of Sotah in which it says (ibid 22): “v’amrah ha’ishah amen amen,” to allude that “one who is practiced in answering amen acquires for himself kedushah.” This is because when he answers amen the oneh is meyached the two Names of Hashem, Havay-h and Adnus, which as is known are numerically equivalent to amen.
The Chashukei Chemed stated that in Yerushalayim, they called a certain Yid a “Kadosh,” only because even though he was aged and weak, he made the effort to go to shul to answer amen to brachos.
Sefer Hatzava’ah 21; Semuchim La’ad Naso; Notrei Amen Vol. I p. 157
