Maimonides on One-Law and the Isaiah 56 Imperative for Gentiles to Completely Identify with Israel

The following comes from Rambam's "Letter to Obadiah the Proselyte":

"I received the question of the master Obadiah, the wise and learned proselyte, may the Lord reward him for his work, may a perfect recompense be bestowed upon him by the Lord of Israel, under whose wings he has sought cover.

You ask me if you, too, are allowed to say in the blessings and prayers you offer alone or in the congregation: “Our God” and “God of our fathers,” “You who have sanctified us through Your commandments,” “You who have separated us,” “You who have chosen us,” “You who have inherited us,” “You who have brought us out of the land of Egypt,” “You who have worked miracles to our fathers,” and more of this kind. 

Yes, you may say all this in the prescribed order and not change it in the least. In the same way as every Jew by birth says his blessing and prayer, you, too, shall bless and pray alike, whether you are alone or pray in the congregation. The reason for this is, that Abraham our Father taught the people, opened their minds, and revealed to them the true faith and the unity of God; he rejected the idols and abolished their adoration; he brought many children under the wings of the Divine Presence; he gave them counsel and advice, and ordered his sons and the members of his household after him to keep the ways of the Lord forever, as it is written, “For I have known him to the end that he may command his children and his household after him, that they may keep the way of the Lord, to do righteousness and justice” (Gen. 18:19). Ever since then whoever adopts Judaism and confesses the unity of the Divine Name, as it is prescribed in the Torah, is counted among the disciples of Abraham our Father, peace be with him. These men are Abraham’s household, and he it is who converted them to righteousness. 

In the same way as he converted his contemporaries through his words and teaching, he converts future generations through the testament he left to his children and household after him. Thus Abraham our Father, peace be with him, is the father of his pious posterity who keep his ways, and the father of his disciples and of all proselytes who adopt Judaism. 

Therefore you shall pray, “Our God” and “God of our fathers,” because Abraham, peace be with him, is your father. And you shall pray, “You who have taken for his own our fathers,” for the land has been given to Abraham, as it is said, “Arise, walk through the land in the length of it and in the breadth of it; for I will give to you” (Gen. 13:17). As to the words, “You who have brought us out of the land of Egypt” or “You who have done miracles to our fathers” – these you may change, if you will, and say, “You who have brought Israel out of the land of Egypt ” and “You who have done miracles to Israel.” If, however, you do not change them, it is no transgression, because since you have come under the wings of the Divine Presence and confessed the Lord, no difference exists between you and us, and all miracles done to us have been done as it were to us and to you. Thus is it said in the Book of Isaiah, “Neither let the son of the stranger, that has joined himself to the Lord, speak, saying, ‘The Lord has utterly separated me from His people’” (Is. 56:3). There is no difference whatever between you and us. You shall certainly say the blessing, “Who has chosen us,” “Who has given us,” “Who have taken us for Your own” and “Who has separated us”: for the Creator, may He be extolled, has indeed chosen you and separated you from the nations and given you the Torah. For the Torah has been given to us and to the proselytes, as it is said, “One ordinance shall be both for you of the congregation, and also for the stranger that sojourns with you, an ordinance for ever in your generations; as you are, so shall the stranger be before the Lord” (Num. 15:15). Know that our fathers, when they came out of Egypt, were mostly idolaters; they had mingled with the pagans in Egypt and imitated their way of life, until the Holy One, may He be blessed, sent Moses our Teacher, the master of all prophets, who separated us from the nations and brought us under the wings of the Divine Presence, us and all proselytes, and gave to all of us one Law. 

Do not consider your origin as inferior. While we are the descendants of Abraham, Issac, and Jacob, you derive from Him through whose word the world was created. As is said by Isaiah: “One shall say, I am the Lord’s, and another shall call himself by the name of Jacob” (Is. 44:5).”"