"The highly honored master & rabbi, Nathanel (of blessed memory) bar Fayyumi."
(his standing, according to Rambam, Responsa of Rambam II, p.i, Lichtenberg edition)
[my emphasis; notes in the text hyperlinked below]
Know then, my brother, that nothing prevents God from sending unto His world whomsoever He wishes whenever He wishes, since the world of holiness sends forth emanations unceasingly from the light world to the coarse world to liberate the souls from the sea of matter — the world of nature — and from destruction in the flames of hell. Even before the revelation of the Law He sent prophets to the nations, as our sages of blessed memory explain, "Seven prophets prophesied to the nations of the world before the giving of the Torah: Laban, Jethro, Balaam, Job, Eliphaz, Bildad, and Zophar." And again after its revelation nothing prevented Him from sending to them whom He wished that the world might not remain without religion. The prophets declared that the other nations would serve Him from the rising of the sun to the setting thereof : "For from the rising of the sun to the setting thereof great is my name among the nations." And further, "For unto me shall every knee bend and every tongue swear fealty [which is done as a covenant; there are stipulations one follows in doing so]." Us He chose and exalted from among the nations, not because of our surpassing excellence but because of His regard for our fathers Abraham, Isaac, and Jacob: "Not because you were more numerous than all the other nations did the Lord desire you and choose you — for you are the least among the nations — but because of the Lord's love for you and to keep the oath which He swore unto your fathers ;" I love you,' saith the Lord. And they say, 'In what respect hast thou loved us.' I loved Jacob.'" God chose us, revealed unto us His laws and ordinances, and imposed upon us a weighty task such as He did not impose upon anyone before or after us, in order thereby to make our reward great: "And the Lord commanded us to carry out these statutes for our good throughout all times, to keep us alive, even as we are this day,"" "It shall be accounted righteousness unto us to do all these statutes;'" "He declared His words to Jacob, His statutes and judgments to Israel. He hath not done so to any other nation'"' "And ye shall be unto me a peculiar treasure from among all people;" "For which is the great nation unto whom God is near?"; "For which is the great nation which has righteous statutes and judgment?'"

And he swore by the tongues of the prophets — peace be upon them ! — that if we forsook His law and the duty He has imposed upon us that He would rule over us with force " 'As I live', saith the King, Lord of Hosts, 'I shall rule over you with a strong hand and with an outstretched arm.' ". The Koran mentions that God favored us, that He made us superior to all other men: "O children of Israel, remember my favor wherewith I showed favor unto you ; and that to you above all creatures have I been bounteous ;" and further, "I have made you excellent with a settled decree, it is not a rumor." He speaks after this manner in many verses and also to the effect that the Torah has not been abrogated. This contradicts what they assert because of the power they exercise over us, because of our weakness in their eyes, and because our succor has been cut off...

It is further said, "God desireth to declare these things unto you and direct you according to the ordinances of those who have gone before you." That indicates that Mohammed was a prophet to them but not to those who preceded them in the knowledge of God. And he said, "O People of the Book, He shall not accept a deed of you unless ye fulfill the Torah." And again, "If there is any doubt concerning what I reveal unto thee, then ask those who received my Book before thou didst." This indicates that He would not have commanded him to ask concerning the Book had He annulled it. And if they say, "Lo, our Book abrogates your Book, just as your Book abrogates the Book of Abraham," we reply, "That is not true. On the contrary, we uphold the religion of our father Abraham, and especially circumcision which God made incumbent upon him, according to the passage, "For I know him, that he will command his sons and his house after him,' "' etc.

When God sent Moses al-Kalim with the Torah to the Children of Israel they were six hundred thousand. And God made incumbent upon them what He had made incumbent upon Abraham, but to those duties he added what the times required. But He did not annul the Law of Abraham. On the contrary, in a number of passages Moses al-Kalim calls upon God in His name and in the name of Isaac and Jacob...Thus He obligated the Children of Noah to observe only seven laws. This was because the Noachides were few in number and because the pre-Abrahamic period could not bear more laws. When Abraham appeared God enjoined upon him the observance of various additional laws. He carried out the Law of Moses, taking it as a duty upon himself before it was binding. Likewise, when God imposed duties upon the Children of Israel to be performed in the Land of Syria they assumed these duties before they entered the land as a mark of obedience to their Creator...They could have believed in them without doing them, but they did them that they might believe in them. Similarly, Adam, Noah and Abraham. In reference to Adam we read, "And he placed him in the garden of Eden to till and to guard it."' In the case of Abraham we read, "Because Abraham has hearkened to my voice and observed my charge.'" Similarly, God has made incumbent upon us in the days of the Messiah all that pertains to
sacrifices and other things, though there never appears again that which was explained by the tongue of the prophet Ezekiel concerning the offerings and the building of the Temple. And similarly, the gathering of all the nations unto the Messiah, according to the passage, "All the nations shall be gathered unto it, the name of the Lord.'"'

When they say unto us ; "That was incumbent upon you in the time of Moses but not otherwise; when other times came ye abrogated your Law and entered into another", we reply, "Know that God commanded that all the people should serve according to the Law; and He permitted to every people something which he forbade to others, and He forbade to them something which He permitted to others, for He knoweth what is best for His creatures and what is adapted to them even as the skilled physician understands his patients, and even more since the physician prohibits food and nourishment to whomsoever he wishes, and permits them to whomsoever he wishes, and they dare not contradict him in anything, because they yielded themselves up to him 'in good faith, sincerity and justice, the more reason that the Creator, to whom nothing can be compared, who is above all comparison or thing compared above the intelligent and the intelligible, understands the well-being of all His creatures; their reckoning and their punishment are entirely in His hands. Whomsoever He wishes He punishes, whomsoever He wishes He rewards, and whomsoever He wishes He compassionates. No hand is above His, and neither interdict nor decree are necessary against the one whom He regards worthy of being punished and cut off from the Divine mercy. All are in His service. His mercy gives them ample sustenance in this world and in the world to come, as it is written, "Good is the Lord to all and His mercies are over all His creatures." It is obligatory upon us to observe what is in our hands, that which we have learnt concerning Him [Torah/Mesorah is a limited data set - limited only by God; it is not necessarily an exhaustive account or source for others, can only be considered such for Jews], that we disobey not one of the Divine Laws and become as Holy Writ hath it, "They made me the keeper of the vineyards but my own vineyard have I not kept."

To the service that all His creatures owe Him as He wishes and how He wishes there is a very nice example :
A king required the services of the people of his city to build a palace. Some of them were architects, some were carpenters, some decorators, some mortar mixers, and some smiths. Of these some zealously carried out the command of the king, some were lax, and some deserted the king's service. The king had maintained all of them. The manner of their service was made known to the king and he waited until he sent for them and called them to account for the manner in which they had carried out his command. He rewarded all those who have done well in their trade more than they deserved, and punished all those who misbehaved in their trade and repentance was of no avail to the penitent if good works had not preceded him. Similarly, the Creator — magnified be His praise! — knows the ruin of this world and the abode of the future world. He therefore sends prophets in every age and period that they might urge the creatures to serve Him and do the good, and that they might be a road-guide to righteousness. The one who was saved was saved through his understanding; and the one who perished perished with full understanding. It is incumbent, then, upon every people to be led aright by what has been communicated to them through revelation and to emulate their prophets, their leaders and their regents. Not one people remained without a law, for all of them are from one Lord and unto Him they all return. All call unto Him, all turn their faces unto Him, and every pious soul is translated to Him, as it is written, "And the spirit returns unto God who gave it." We shall follow this subject with the mention of the world to come in the chapter after this — please God ! — since after the Messiah there is nothing save that — truly God knows better and is wiser.

Nathanael Ibn al-Fayummi Bustan al Ukul p.103-108