A Jew is a Jew no matter what he does he always remains a Jew, and a Jew cannot leave the Jewish nation although he converts to a different religion g-d forbid.
As the Maimonides writes in his famous letter to the Jews of yemen “…as the torah, one cannot run away from it, every descendant of Jacob cannot leave the torah neither him nor his offspring , with consent or without consent , and (being that one cannot leave Judaism one always has a complete responsibility towards torah and mitzvois and therefore) one will be punished for all of his transgressions and although he has transgressed severe transgression he also bares responsibility for the smaller wrongdoings as well” .
[1]
Nonetheless if one officially left the Jewish nation being that this a very severe act when he repents and returns , there is discussion amongst halachic authorities is he required to do anything in addition to the actual keeping of torah and mitzvois which he is to keep.
[2]
Some hold that one who converted out has actually in some ways left the jewish nation and is therefore obligated to go through a process which is like conversion (haircut, circumcision if needed and mikvah), other halachic authorities hold that one is not required to immerse in the mikveh, and it is sufficient that he does teshuvah and decides to return to hashem.
[3]
While others hold that being that he has not left the jewish nation and he is totally connected to it there is no obligation of conversion as he is still a complete Jew yet he is to purify himself by immersing in the mikvah (unlike conversion the mikveh does not need to be before a beis din)
[4] and thereby remove the impurity which he received by converting to a different faith.
[5]
The halacha follows that although one is a full Jew nonetheless one is to immerse in the mikveh (in truth one is to immerse in the mikveh three times one time to purify himself from the impurity of sin, a second time to purify himself from the sin of eating non kosher food which contaminates ones mind and soul, and a third time to sanctify themselves with the sanctity of the Jewish people and to bestow upon himself the sanctity and holiness of torah and mitzvois , a male is to add a fourth immersion in order to purify himself from impurity which is related to keri (impurity which results from one’s relationships etc…) and after that one then to officially accept upon himself the “divire chavirus” to act like a proper Jew learning torah and following the mitzvois.
[6]
Some add the importance of fasting to remove the stain from his soul yet one should on a case to case basis seek the advice of a rabbi who knows them and another important point is to feel bad about what happened and totally distance themselves from the gentile pastors and priests and not to have any discussion with them at all , and totally return full heartdly to hashem .
[7]
yet even if they don’t, they are still obligated in all the mitzvois as mentioned above.
In conclusion one is to return to Judaism and in practice they are to immerse in a mikvah and accept the fundamentals of Judaism in front of three rabbis
