Gemara

(R. Avahu): The Halachah follows R. Yehudah. One may not interrupt between va'Yomer and Emes v'Yatziv.

(R. Avahu): He learns from "va'Shem Elokim Emes."

(R. Avahu): One says "Emes" twice.

(Rabah): He says it only once.

A man in front of Rabah said "Emes" twice.

Rabah: He was overcome by a craze that made him repeat "Emes."

Shabbos 119b (R. Chanina): Amen is the acronym of 'Kel Melech Ne'eman.'

Rishonim

R. Yonah (8a DH Chozer): The argument about repeating Emes refers to one who interrupted (Birkos Keri'as Shema) due to fear or honor after saying Emes, or he finished Keri'as Shema before the Shali'ach Tzibur, and said Emes after Ani Hash-m Elokeichem, and waits for the Shali'ach Tzibur to begin, to say (Emes v'Yatziv) with him. The Halachah is, he need not repeat Emes. Rather, he begins from v'Yatziv, or from where he stopped.

Hagahos Maimoniyos (Hilchos Berachos 1:8): (The Medrash) Yelamdenu says that there are 248 words in Keri'as Shema, corresponding to the limbs. The Ramach says that there are 248 only if one says Kel Melech Ne'eman. R. Moshe says that one should not say it. Rather, one answers Amen in place of it. This is only if he answers to the Shali'ach Tzibur, but not if he blessed by himself. This is the custom in Sefard.

Poskim

Shulchan Aruch (OC 61:3): There are 245 words in Keri'as Shema. In order to complete 248 words, which correspond to a man's limbs, the Shali'ach Tzibur concludes Hash-m Elokeichem Emes, and repeats it aloud.

Beis Yosef (DH v'Rabeinu, citing Mahari Avuhav): Orchos Chayim says that some repeat "Lihyos Lachem lEi'lokim." Even though an individual does not repeat, like it says in the Gemara, since he hears from the Shali'ach Tzibur, Shome'a k'Oneh (hearing is like answering). Most places do not do so. Rather, the Shali'ach Tzibur repeats Hash-m Elokeichem Emes.

Beis Yosef (ibid.): Where does the Gemara say so? Perhaps he refers to 'he does not repeat Emes", and this is only for an individual. Some question the custom due to this. The Halachah follows Rava, who says not to repeat Emes. A man said "Emes Emes" in front of Rava, and Rava said that this was improper. This connotes that even a Shali'ach Tzibur does not repeat Emes. We can distinguish repeating Hash-m Elokeichem Emes from saying Emes Emes. R. Yonah says that he does not repeat Emes even though he interrupted with other words. It is not common for a Shali'ach Tzibur to interrupt for fear or honor. Rather, he discusses an individual, who surely does not repeat Hash-m Elokeichem Emes. This is why Rava said that the man erred. One may repeat Hash-m Elokeichem Emes.

Beis Yosef (ibid.): The case with Rava connotes that their custom was not to repeat Hash-m Elokeichem Emes. We can say that Rava meant that he should have repeated Hash-m Elokeichem Emes. Alternatively, Rava used to say Kel Melech Ne'eman, like the ancient custom, therefore one need not repeat any word. Rava agrees that one who does not say Kel Melech Ne'eman, like our custom, must repeat Hash-m Elokeichem Emes.

Beis Yosef (DH v'Nimtza): Sefer ha'Pli'ah says that the Shali'ach Tzibur repeats Hash-m Elokeichem Emes. A Shali'ach Tzibur did so in Mitzrayim, and great Rabanim rebuked him. However, the Zohar says like Sefer ha'Pli'ah, so one should follow the ancient custom, which is based on it.

Magen Avraham (1): Where the custom is not to repeat, we do not protest. Asarah Ma'amaros says that the Shali'ach Tzibur does not say quietly Emes. An individual says Emes, but does not repeat Emes, so he has exactly 248 words. If (a Shali'ach Tzibur) would say quietly Emes, and repeat Hash-m Elokeichem Emes, he would have 249 words. This explains the Amora'im's argument about repeating Emes. R. Moshe Alashkar says to say Emes also the first time, to avoid separating (Hash-m Elokeichem from Emes). The second Emes does not count towards the number of words. It is part of Birkas Emes v'Yatziv.

Gra (DH Mesayem): The Gemara connotes that an individual concludes with Emes, but does not repeat it. The Shali'ach Tzibur initially concludes without saying Emes. (He says it only after repeating "Hash-m Elokeichem.") Surely Emes counts towards 248 words, even though it is part of the next Berachah. A proof is that the 15 Vovim (in Emes v'Yatziv) can count towards 248, even though they are part of the next Berachah, and all the more so according to the Rema (below, that the Vovim count like "Hash-m Ad-nai Emes"), which is primary. What the Beis Yosef wrote is very difficult.

Mishnah Berurah (8): The custom is that the Shali'ach Tzibur says Emes also the first time. The Gra says that this is not primary.

Sha'arei Teshuvah (2): The Tzibur should not repeat Hash-m Elokeichem Emes. If the Shali'ach Tzibur says this before one finished Shema, he listens and then finishes Shema. This is better than having intent in the Vovim.

She'elas Rav (ha'Gaon R. C. Kanievsky Shlita, 2:1:40, 2:5:25): One may say Kel Melech Ne'eman b'Tzibur, but l'Chatchilah one should not start Emes v'Yatziv before the Shali'ach Tzibur, even to enable saying Shem Hash-m with more Kavanah. Rather, he waits to hear Hash-m Elokeichem Emes from the Shali'ach Tzibur. Sha'arei Teshuvah holds that even b'Di'eved one can be Yotzei through the Shali'ach Tzibur only if he already began Shema.

Az Nidberu (8:41:2): Perhaps one can be Yotzei only if he already began Parshas Tzitzis.

Mevakshei Torah (166:33, citing ha'Gaon R. S. Z. Auerbach Ztz"l): One can be Yotzei even if he did not yet start Shema.

Kaf ha'Chayim (15): Many say that one may be Yotzei through the Shali'ach Tzibur before he finished Keri'as Shema. However, the Ari Zal gave reasons for the number of words in each Parshah. One who is Yotzei while in a Parshah ruins the effects it should have. Rather, he should repeat the words himself. All the more so it is improper to say Kel Melech Ne'eman. Even though we do not have the Kavanos, we must say according to the proper order.

Kaf ha'Chayim (11): Since there is no contradiction to the Gemara, we follow the Kabalah. Even if the Shali'ach Tzibur initially said these three words aloud, he repeats them aloud.

Kaf ha'Chayim (12): Machazik Berachah says that the Tzibur waits to say Emes with the Shali'ach Tzibur. Chesed la'Alafim disagrees, for one may not interrupt between Hash-m Elokeichem and Emes. This is wrong. We forbid interrupting only through speech! Also, Shome'a k'Oneh. Hearing "Hash-m Elokeichem" is like saying it himself.

Rema: Everyone is Yotzei through this, since he hears these three words from the Shali'ach Tzibur. It one wants to say them with the Shali'ach Tzibur, it is not Asur.

Magen Avraham (2): One may not repeat just Emes, with the Shali'ach Tzibur, for this is like saying Shema Shema. Keneses ha'Gedolah says that we do not silence him. This is only if he paused in between.

Kaf ha'Chayim (13): If one is unsure whether the Shali'ach Tzibur knows to intend to be Motzi people through these words, he can say the words himself. The Ari Zal did so when he prayed alone.

Kaf ha'Chayim (14): One may repeat these words also in Keri'as Shema Al ha'Mitah.

Shulchan Aruch (ibid.): If one says Shema alone, he intends for the 15 'Vovim' in (the start of) Emes v'Yatziv. They have (in all) the Gematriya 90, which is like three times the Gematriya of Hash-m's name Yud-Kei Vov-Kei (26) and four for the four letters, making 30 in all.

Magen Avraham (3): In Ma'ariv one intends for the "Vov" in ve'Emunah. Asarah Ma'amaros says that he intends that Emes count like three words.

Machatzis ha'Shekel: A 'Vov' is spelled Vov Aleph Vov. The sum of the Gematriyos of the letters from Aleph to Vov is 21, like the name "Echyeh" (with a Hei, and not with a Ches). The letter Aleph is a Vov with two Yudim. Its Gematriya is 26, like Yud-Kei Vov-Kei. The letters of Emes are the first, middle and last letters of the Aleph Beis. (Including the five final letters, there are 27 letters. Mem is the 14th, exactly in the middle - PF.) They allude to "Ani Rishon va'Ani Acharon umi'Bil'adai Ein Elokim."

Mishnah Berurah (12): In Ma'ariv, one intends that Emes count like three words.

Kaf ha'Chayim (18): Some count the Vovim on their fingers to ensure that they not miss any.

Rema: Another reason is that the Gematriya of 15 Vovim, plus one for the recitation, is 91, the combined Gematriya of the way we pronounce Hash-m's name (i.e. like Aleph Dalet Nun and Yud, which add to 65) and the way it is written (26), so it is as if he said Hash-m Ad-nai Emes.'

Gra (DH she'Olim): The Rema is primary. According to the Mechaber, 13 Vovim should suffice (78 is three times 26. We need not add an extra four for the four letters.) Also, Emes is a Hefsek in between (the Vovim and Keri'as Shema, since Emes itself is not part of the 248). Also, according to the Rema, just like (a Shali'ach Tzibur says) Hash-m Elokeichem Emes, also here (it is as if an individual says) these two names (Hash-m Ad-nai, and Emes - Damesek Eliezer).

Rema: Some say that am individual who reads Keri'as Shema says Kel Melech Ne'eman beforehand to complete the 248 words. They are in place of the Amen that one should answer to the Berachah before Shema, ha'Bocher b'Amo Yisrael b'Ahavah. This is the custom. In any case one who reads with a Tzibur should not say Kel Melech Ne'eman, just he answers Amen after the Shali'ach Tzibur's Berachah. This is the custom.

Beis Yosef (DH Yesh): Some say Kel Melech Ne'eman before Keri'as Shema, and some add Amen to it. They explain that Keri'as Shema lacks four words from 248. This is not precise. There are 12 words in the verse Shema Yisrael... and Baruch Shem, another 42 until uvi'Sh'arecha, 122 in v'Hayah Im Shamo'a, and 65 in Parshas Tzitzis. This is 245 in all. We lack only three words! Rather, the Amen is for the Berachah "ha'Bocher..." The Agur says that in Ashkenaz and France they say Kel Melech Ne'eman, but not Amen. Maharik (42) agrees. The Agur brings that Rashi and R. Yosi ha'Chasid wrote to say Kel Melech Ne'eman. Hagahos Maimoniyos says not to say it. The Ramah rejected the custom, for it is not in the Mishnah or Gemara. I say that it is an ancient custom. Later, it was abolished, in order to avoid a Hefsek between ha'Bocher... and Shema. In place of it, they enacted that the Shali'ach Tzibur repeats Hash-m Elokeichem Emes. Sefer ha'Tikunim and the Zohar say so. This is the custom of Bnei Sefard. When one reads Shema properly, every limb receives one word and is healed through it. One who reads Shema alone should intend for the 15 Vovim, but even so, this is Me'uvas Lo Yuchal Liskon.

Taz (2): The Bach says that the custom was Batel. We do not say Kel Melech Ne'eman. We rely on the Zohar; the Shali'ach Tzibur repeats Hash-m Elokeichem Emes.

Magen Avraham (4): The Maharshal did not want to interrupt to say this.

Gra (DH v'Nireh): The Ramah objected because it interrupts. Even answering Amen to one's own Berachah is improper for this reason! Even Baruch Shem Kevod... (Pesachim 56a suggested not to say it, for it is not in the Torah, so perhaps it is a Hefsek). Tikunim connote that it is a Hefsek because one can hear (three words) from the Shali'ach Tzibur. This does not apply to an individual. Even though an individual can intend for the Vovim, this is Me'uvas Lo Yuchal Liskon.

Gra (DH Rak): Amen is in place of Kel Melech Ne'eman, like it says in Shabbos. See Tosfos there (DH Amar; one must think Kel Melech Ne'eman when saying it). The Rema holds like the Rosh, who says that Amen after the Shali'ach Tzibur is not a Hefsek.

Mishnah Berurah (15): One may not learn from here that if one blessed on a food together with someone else, he may answer before eating. The Rema holds that the Berachah was not enacted specifically for the Mitzvah of Keri'as Shema.

Mishnah Berurah (16): It is better to finish together with the Shali'ach Tzibur, so he will not need to answer Amen.

Kaf ha'Chayim (19): The Ramah says that it is a Hefsek between the Berachah and Keri'as Shema, it is Hash-m's name in vain, and it is an improper addition to Keri'as Shema.