When a non-jew converts to Jusaism, must he still do teshuva from past sins, or does the "reborn" concept stretch far enough to consider him "sinless"?
From Rambam, Laws of Kings and their Wars, Chapter 10:
בן נוח שבירך את השם, או שעבד עבודה זרה, או שבא על אשת חברו, או שהרג חברו, ונתגייר--פטור. הרג בן ישראל, או שבא על אשת ישראל, ונתגייר--חייב; והורגין אותו על בן ישראל, וחונקין אותו על אשת ישראל שבעל--שהרי נשתנה דינו.
A non-Jew who commits a capital offense and then converts: if the capital offense did not affect a Jew (e.g. idol worship, cursing G-d's name), then the Jewish courts don't carry out any punishment against the new convert. But if a non-Jew murdered a Jew, or was involved with a Jewish married woman, then he is held accountable even after conversion.
Interesting. So apparently, as you've indicated, it's not entirely "clean-slate."
The Talmud has stories of people who'd done horrible things against the Jews rethinking their lives, running off, and converting. Apparently we accepted them for conversion; how they work out their previous lives and regrets is between them and G-d.
Based on the Gemara that says he is like a child KiKatan Shenolad Dami it would seem he has a completely clean slate like a child.But my original conclusion is wrong as pointed out by Yahu.
In fact even the question of familial relationships is a Machlokes in a Gemara in Yevamos between Reish Lakish, who says if he had a child prior to his conversion he is still obligated, because he resembles a katan shenolad so the original Children have nothing to do with him.Then there is Reb Yochanan who says it makes no difference as the reality is he has children.
They have the same argument about the status of a child born after a geirus in regards to a firstborn getting the double portion of inheritance. Reb Lakish says the child born after birth is a Bechor Reb Yochnan says no.
Another gemara that disproves my original point is in Sanhedrin it says if someone did an Aveirah and was converted he is still responsible for his sin. The Chavas Yair brings an example of a ger who had stole from a Jew prior to conversion The question asked was must he return the item? the Chavas Yair said yes again proving "no clean slate", Another interesting Machlokes on this topic is a women who ran away with a gentile and then returned to Judaism and the gentile converted can she now marry him (If her original husband died,or divorced her) normally we say she can not marry the person she cheated with or continue living with her husband The Rabbenu Taam as explained by the Mordechai says she is allowed to marry the man she ran away with as the Geirus makes a New man Kikoton Shenolad Dami MOST DISAGREE this ruling and again it seems "No clean Slate"
In fact to knock the Final nail in the coffin of my original thought process the gemara in Yevamous says one of the reasons Converts suffer after converting is because they failed to keep the Seven Noachide laws earlier again disproving The Clean slate. Source