Chizkuni on Numbers 5:8 refers to God as the "Father of the convert". Do any other, earlier sources convey this idea?
The Rambam’s Letter to Ovadiah the Ger
“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).”
The Full Letter
Maimonides, Letter to Ovadia to Convert
Thus says Moses, the son of Rabbi Maimon, one of the exiles from Jerusalem, who lived in Spain:
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).
"The story of the Ger Tzedek took place in 5509 (1749), and the Vilna Gaon (the Gra), who was then about twenty-nine years old, knew the Ger Tzedek and had secret ties with him.
They say that the Gra once came to visit the Ger Tzedek in prison and found him worried. Seeking an explanation, he said to the Ger Tzedek, "You should be happy, because in a few days you'll reach a very high madreigo, to sanctify Heaven's name in public, like the level of the tana hakodosh Rabbi Akiva."
The Ger Tzedek answered the Gra that he was worried for a different reason -- he did not have zechus ovos, for his father and mother were gentiles who did not believe in the Creator of the world. The Gra comforted him and said, "Hakodosh Boruch Hu says, `I am first and I am last;' Hakodosh Boruch Hu is the father of all those who do not have yichus ovos."
HoRav Boruch Ber Leibowitz of Kaminetz told the story of the Gra's visit as follows: The Gra once visited the Ger Tzedek, the tzaddik Reb Avrohom, and found him crying. The Gra wondered why; after all, you are going to sacrifice your life al kiddush Hashem. Why are you crying? You should go happily.
The Ger Tzedek answered that he was not crying because of that. He was crying because he was not zoche (did not have the merit or chance) to put down roots in Am Yisroel (the Jewish people), for he had no father or son in Am Yisroel .
The Gra said, "We find in the medrash (midrash in Hebrew) the posuk (verse),
I am first and I am last etc.' (Yeshaya 44:6) I am first for I have no father; I am last for I have no brother; And besides Me there is no power for I have no son' (Shemos Rabba chapter 29, 5. see also Yalkut Hameiri, Yeshaya 44). The words seem questionable. Why does it have to say something that everyone knows?
"However," Rabbenu answered, "
I am first' for someone who has no Jewish father and came to bask in My shade; I am last' for someone who doesn't have a brother; `And besides Me there is no power' for someone who doesn't have a son. I am better for him than ten sons.""
Im considering this earlier sources because this idea is based off the midrash.