"Their blessings are nothing, and one should not respond amen. ...
One who hears a Jew recite any of the blessings-even though he did not hear the entire blessing from beginning to end, even though he is not required to say this blessing himself-must respond amen. But if the one who recites the blessing is an Apikoros or a Samaritan or a minor [who is merely practicing] or an adult who alters the fixed form of the blessing, one does not respond amen.
Moshe Isserles adds: "One responds amen to an idolator if one heard the entire blessing from his mouth." These teachings are based on Mishna Berakhot 8.8, the Babylonian and Jerusalem Talmuds ad loc, and Tosefta Berakhot 3.26 and 5.21.
In context, it appears clear that the Shulhan Arukh prohibits saying amen because any blessing by a Samaritan or heretic is a berakha levatala, a blessing in vain.
Let us begin with the Mishna: "One responds amen to a Jew who blesses; but one does not respond amen to a Samaritan (kuti) who blesses, unless one hears the entire blessing." (38) It would appear obvious from the plain meaning of the Mishna that the reason to refrain from answering a Samaritan's (39) blessing is that this person, whose Jewish bona fides are in question, might be worshipping idols.
If we observe a Samaritan mumbling something in prayer or at the table, it only makes sense to refrain from answering amen until one knows exactly what he or she is affirming.
For the sake of good will, one might want to say amen when she recites blessings.
On the basis of our Mishna, I cannot see how one would refrain from answering amen. Here, the person whose usual practice of Judaism is anathema to us, nevertheless said a proper--and obligatory!--blessing of God.
Once I have ascertained that this woman's blessing expresses nothing but admirable religious intentions, how could I rob God of my whole-hearted amen? Did God cease to be the one who "brings forth bread," just because a religious deviant said so?
Although there are variations among the textual witnesses to these passages, the basic idea of all the versions is that a Jew should respond amen whenever someone blesses God, as long as this person is not reciting an improper blessing.
To be sure, the rabbis would not respond amen if a gentile were to drink a libation to Mars.
In our case, in standard printed editions of the Mishnek Torah (Laws of Blessings 1.13), Maimonides rules that one never responds amen to blessings said in vain, including those said by gentiles, Apikorsim and Samaritans.
Elijah, the Gaon of Vilna, could explain the Rambam's strange ruling only as a scribal error: "These words [of R Yosef Karo] indicate that one should not respond amen to a Samaritan, even if one heard the entire blessing.
His brief explanation for why one should respond amen to the complete blessing of a Samaritan or non-Jew is resonant with religious meaning, and it bears examination: "Since one hears that this [Samaritan] has blessed God upon this item-even though he does not even know what God is, for he thinks that his idol is the Creator!-nonetheless, since his intention was toward God, and we hear the entire blessing from his mouth, we respond amen." (49)
Rather, through the religious act of saying amen, the Tosafot Yom Toy embraced a wide range of human beings who direct their prayer to God.