We will begin this response by establishing that this is a fallacious question. The questioner demands a "Sola Scripture" proof of post-Sinai commandments for Gentiles (Non-Jews), and in lieu of that, he claims there is no such religion given by G-d. Yet, he refers to something called a "Sinai Covenant." An obvious part of the Sinai Covenant is that G-d made a covenant with the Jewish people to follow a set of Jewish commandments, and those Jewish commandments constitute the basis of observing the Jewish religion. However, there is no rabbi who can use Scripture to define how the majority of the Jewish commandments are to be observed:
You will not find anywhere in Scripture a clear definition of the things that Jews are to abstain from doing on their Sabbath, or what it means to have words of G-d as "a sign on your hand" and "frontlets between your eyes" (the commandment for wearing Tefillin), or how a kosher species of animal or bird is to be slaughtered in order to make it permissible for Jewish consumption. All of these Jewish commandments, and hundreds more, are defined only in the Oral Torah that G-d gave to Moses on Mount Sinai. The same applies to the other part of the Sinai Covenant, which is the Noahide Code. The failure and falsity of the "Sola Scriptura" doctrine, which was invented by man-made religions that tried to distort the true meaning of the Sinai Covenant, is very well explained in the following series of 6 video segments that were produced by an observant Noahide, and which should be learned thoroughly by everyone:
https://asknoah.org/video/oral-torah-part-1
Nevertheless, the question will now be answered on its level:
The essential point the questioner is missing, and which his citations from Isaiah do not disprove at all, is that there are commandments from G-d for Non-Jews that they have to follow AFTER Mount Sinai, in order to be righteous. If they violate those commandments, they are liable to be punished by G-d if they are aware of the forbidden nature of the act. Here are 7 cases in point, based on Tanakh (the Hebrew Bible):
(1) Idolatry:
This is blatantly obvious, because it says throughout the Books of Exodus through Deuteronomy that after Mount Sinai, G-d decreed punishment upon the people of the Canaanite nations on account of their idolatry, which G-d refers to as "their wickedness" and "their abominations." If they were not breaking any commandment by worshiping idols, G-d would have had nothing to punish them for on that account, and He would not have called their idolatrous acts "wicked" and "abominations." See:
Deut. 9:5 - "...because of the wickedness of these nations, the L-rd your G-d drives them out from before you..."
Deut. 20:17-18: Rather, you shall utterly destroy them: The Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivvites, and the Jebusites, as the L-rd, your G-d, has commanded you; so that they should not teach you to act according to all their abominations that they have done for their gods..."
For a specific example of a Non-Jew who righteously abandoned idolatry, see the account of the Aramean Non-Jewish general Naaman in II Kings 5:17-18, who was miraculously cured by Elisha:
"And Naaman said, 'Now, if only your servant be given a load of earth as carried by a team of mules [to fill an altar with earth from G-d's Holy Land], for your servant will no longer offer up a burnt-offering or a sacrifice to other deities, but to the L-rd. For this thing may the L-rd forgive your servant; when my master [the Aramean king] comes to the temple of [his idol] Rimmon to bow down there, he leans on my arm, so I must bow in the temple of Rimmon; when I bow down in the temple of Rimmon, may G-d forgive your servant for this thing."
If there was no sin in a Non-Jew bowing to an idol, Naaman would not have needed to ask for any forgiveness from G-d on that account. He was only letting Elisha know that he would avail himself of the Torah Law that allows a Non-Jew to bow to an idol in order to save his life, because his master the king would have him killed if he refused to do that.
Also it says in Isaiah 45:20-22 about the Non-Jews of the world, "Assemble and come, approach together, you survivors of the nations, who do not know, who carry about their graven wooden image, and pray to a god who cannot save... There is no other god besides Me; there is no righteous god besides Me and no savior other than Me. Turn to Me and be saved, all the ends of the earth, for I am G-d, and there is no other."
This proves that post-Sinai, the Non-Jews can only be saved by renouncing their idolatry and turning to the One G-d alone.
See also the introduction to #7 below.
(2) Forbidden relations:
This is also blatantly obvious, because it says throughout the books of Exodus through Deuteronomy that after Mount Sinai, G-d sentenced the people of the Canaanite nations death on account of their committing forbidden sexual relations. If they were not breaking any commandment by committing specific deviant sexual relations, G-d would have had no reason to punish them on that account, nor to call their deviant sexual acts "iniquity" and "abominations."
See Leviticus 18:24-25,27 " ... for through all of these [aforementioned sexual acts] the nations that I expel before you [the Jewish people] became contaminated; and the land became contaminated, and I recalled its iniquity upon it....For the inhabitants of the land who are before you committed all these abominations..."
(3) Theft
G-d decreed destruction upon the people of the Non-Jewish city of Nineveh on account of their "wickedness" -
Jonah 1:2 - " Arise, go to Nineveh, the great city, and proclaim against it, for their wickedness has come before Me."
The king and the people of Nineveh knew that their sin of "wickedness" was robbery, and they righteously repented and made restitution for that sin. If robbery was not commanded by G-d to be prohibited for Non-Jews, they would not have come up with this idea as the reason why G-d was about to punish them:
Jonah 3:7-10, " And he [the king] caused it to be proclaimed and published throughout Nineveh: ... they shall call mightily to G-d, and everyone shall repent of his evil way and of the robbery which is in their hands. Whoever knows shall repent, and G-d will relent, and He will return from His burning wrath, and we will not perish. And G-d saw their deeds, that they had repented of their evil way, and the L-rd relented concerning the evil that He had spoken to do to them, and He did not do it."
(4) Murder
The Torah Law after Mount Sinai is that in the Land of Israel, the Jews are commanded to establish cities of "refuge," to provide refuge for three types of people if they kill someone accidentally. The "refuge" provided is safety from being killed by the blood redeemer, who has permission to kill the person who killed his close relative -- if the killer is not brought to trial.
From Deut. 35, the three classes of people who are granted safety in a city of refuge are as follows: "For the Children of Israel, and the proselyte, and the resident among them shall these six cities be a refuge, for anyone who kills unintentionally to flee there."
The "resident" is not a "Child of Israel" (a born Jew), nor a righteous convert (a proselyte). He is a Non-Jew who has permission to reside in the Land of Israel. For any of these, including the Non-Jew, the following verses apply if the killing was intentional, i.e. murder:
Deut. 35:12,21 "These cities shall serve you as a refuge from an avenger, so that the murderer shall not die until he stands in judgment before the congregation.... if he maliciously struck him with his hand and he died, the assailant shall be put to death [by the court]; he is a murderer; the blood avenger may kill the murderer when he meets him [if a court does not execute him]."
This proves that post-Sinai, G-d decrees death as the punishment for a Non-Jew who kills deliberately, so there must be a post-Sinai commandment from G-d that Non-Jews are prohibited from committing murder.
(5) Blasphemy
[This part has been revised.] Job was a righteous Non-Jew who lived from before the birth of Moses until a little over one year after the Revelation at Mount Sinai. The Book of Job describes how he was stricken with afflictions. When his wife saw his physical suffering, she felt he would be better off dead than alive, so she told him the following:
Job 3:9, "Then his wife said to him, 'Do you still maintain your sincerity? Blaspheme G-d and die!' "
Of course, Job was righteous and he did not blaspheme.
Although this happened before Mount Sinai, it is noteworthy that his wife knew the Noahide commandment that one who blasphemes G-d is liable to death by the Hand of G-d. But there is also an explicit source verse which states after Mount Sinai that blasphemy is a capital sin for Non-Jews: Lev. 23:15, “...ANY man who blasphemes his God shall bear his sin.."
The other point that is relevant here is that Torah testifies that even after Mount Sinai, Job was praised as a righteous Non-Jew. He lived in the Land of Canaan, and he died just before Moses sent the 12 spies to scout the land. Moses was concerned that the merit of Job's righteousness would protect the Canaanites. In Num. 13:17,20 (lit.) - "Moses sent them to scout the Land of Canaan, and he said to them ...'Does it have a tree or not?' " Rashi explains: "Does it have a worthy man who will protect them with his merit?"
When the spies returned, they said, (13:32) "...The land we passed through to explore is a land that consumes its inhabitants..." Rashi explains: "Wherever we passed, we found them 'burying dead' [i.e. in mourning]. The Holy One, blessed is He, intended this for good, to keep them occupied with their mourning so they should not notice [the spies]."
Why were the Canaanites mourning? Because Job had just died, and all the Canaanites knew that they would no longer be protected by his righteousness. This was stated by Caleb, one of the two faithful spies (14:9), "...you will not fear the people of that land for they are [as] our bread. Their protection is removed from them.." Rashi explains: "Their shield and strength, their virtuous ones have died - [namely,] Job, who protected them [with his righteousness]."
(6) Inflicting unnecessary pain or suffering on animals (of which eating meat severed from a living animal is the extreme case)
The story of Bilaam (a Non-Jewish prophet) took place after Mount Sinai. When Bilaam was riding his donkey with the intention to go to curse the Jewish people, his donkey turned aside three times to avoid the angel that G-d sent to stand in his way, which Bilaam could not see. Each time, because he was angry at the donkey, Bilaam beat it with his staff. Then the angel revealed itself to Bilaam and said to him,
Numbers 22:32, "...For what reason did you strike your she-donkey these three times?"
If it was not a sin for a Non-Jew to inflict pain on an animal without a reason for the necessity to do, the angel would not have demanded to Bilaam that he needed to have a valid reason for beating the donkey. If it was not a sin, the angel would not have mentioned it at all. Since beating an animal without a necessary reason is a sin for Non-Jews, how much more so is it a sin for a Non-Jew to cut off a piece of meat from a living animal and eat it. A desire for eating fresh meat does not make it necessary to take the meat from a living animal, because it could be accomplished by killing the animal first, thereby inflicting less pain and suffering.
(7) Laws and Courts
Introduction:
Isaiah ch. 45 is a prophecy from G-d about the righteous Non-Jewish king Cyrus, through whom G-d made know to the Non-Jews of the world that there is no deity except Him:
Isaiah 45:1-3,6,13 "So said the L-rd to His anointed one, to Cyrus, whose right hand I have grasped, to subdue nations before him.... 'I will go before you [Cyrus]... in order that you should know that I am the L-rd Who has proclaimed your name... in order that those [Non-Jews] from east to west would know that there is nothing besides Me; I am the L-rd and there is no other.' I aroused [Cyrus ] with righteousness..."
This proves that post-Sinai, G-d wants all Non-Jews to know that there is no other deity than Him, and one of the ways He accomplishes this is through righteous Non-Jewish leaders like Cyrus, who accept that there is only the One G-d. The prophecy continues:
Isaiah 45:18, "For so said the L-rd, the Creator of Heaven, Who is G-d, Who formed the earth and made it, He established it; He did not create it for 'tohu' [Hebrew for 'chaos' and 'emptiness']; He formed it to be inhabited, 'I am the L-rd and there is no other.' "
This proves that G-d created and established the world so it would be inhabited by people around the world in a settled way, not in a chaotic way that would cause people to have to live apart from each other for their own safety and well-being. The only way to have a safe, settled society, without chaos, is to have laws and courts that are accepted by the people. This proves that G-d wants Non-Jews to have laws and courts that promote societal stability.
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So we have used the Hebrew Bible to establish 7 rules that are commanded by G-d for Non-Jews post-Sinai, and we have cited several righteous or fully repentant Non-Jews from Tanakh who lived post-Sinai: Naaman the Aramean, the king of Nineveh, Job, and Cyrus, each of whom gave a lesson from their righteousness about a particular one of these rules.
These 7 rules are the Commandments for the Children of Noah that were part of the Sinai Covenant established by G-d through Moses, and their source verses are in the Five Books of Moses, as explained in this web page:
https://asknoah.org/7-commandments/locate-sources
The reason why this information is now being distributed world-wide is because G-d gave instructions for the Jewish people to be a Light to the Nations, when He provides the opportunity to do so. This "Light" in our time means informing and encouraging Gentiles about their 7 Commandments, so they can become righteous through them and thereby make themselves and the world ready and receptive for the Messianic Era which is coming very soon. What a blessing it is that more and more Gentiles are turning to the righteous Noahide path and helping greatly with this effort!