A friend of mine asked me from when a Non-Jew was allowed to make giyur (conversion); and on which Biblical verses it was based upon. I guess he wanted to know more about it's origin. So that's why I'm posting this question: From where in the Scriptures (and commentaries) can one trace back the origin for making giyur?
The doctrine of joining/converting to another's religions can be traced to the book of Leviticus from chapters 17-24. Cf. also Isaiah 56:6. In practice, we have it recorded in the book of Ruth whose subject was a convert.
See the thorough background of this topic and its subsequent development in Prof. G. F. Moore's 'Judaism' vol. 1 pg. 323ff.
The children of Israel that went out of Egypt and accepted the Torah were the very first converts.
We read in Deuteronomy 10,19 ואהבתם את הגר כי גרים הייתם בארץ מצרים - You shall love the stranger (convert) for you were strangers in Egypt.
In fact the gemora describes the laws of converts in Yevamos 46a - b אמר ר' חייא בר אבא אמר ר' יוחנן לעולם אין גר עד שימול ויטבול וכיון דלא טביל נכרי הוא - a person isn't a convert until he is circumcised and ritually immerses. (we are commanded to circumcise in genesis 17,12 ובן שמנת ימים ימול לכם כל זכר לדרתיכם- eight days old shall you circumcise for future generations) באבות נמי טבילה הוה מנא ליה - How do we know ritual immersion happened with our fathers מהכא (Exodus 24,8) ויקח משה את הדם ויזרוק על העם וגמירי דאין הזאה בלא טבילה - From here (at Mount Sinai just before accepting the Torah) "and Moses took the blood and threw it on the the people" and we have a tradition that there is no sprinkling without ritual immersion.
From here we see that before the giving of the Torah the Jewish nation were not Jewish yet and they required circumcision which they did before eating the pascal lamb in Egypt prior to leaving as it say Exodus 12,43: ויאמר יהוה אל משה ואהרן זאת חקת הפסח - And G-d said to Moses and Aaron this is the law of the Paschal lamb - and then it says in verse 12,48 וכל ערל לא יאכל בו no uncircumcised can eat it.
They then did immersion as stated above.
From here we learn how all converts embrace Judaism and a sincere acceptance of all the torah and mitzvot via the rabbis is also required like our forefathers said at mount sinai when Hashem offered them the Torah ויקח ספר הברית ויקרא באזני העם ויאמרו כל אשר דבר יהוה נעשה ונשמע - And Moses took the Torah and he recited it in the ears of of all the people, and they said all Hashems words we shall do and listen to.