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Holy Zohar text. Daily Zohar -885
Hebrew translation:

וַתּוֹסֶף לָלֶדֶת אֶת אָחִיו אֶת הָבֶל. בֹּא וּרְאֵה, הֶבֶל הוּא הַלֵּב, וְיֵשׁ שְׁנֵי בָּתִּים בַּלֵּב, לֵב חָכָם לִימִינוֹ הֶבֶל, לֵב כְּסִיל לִשְׂמֹאלוֹ זֶה הֶבֶל, קַיִן הוּא יוֹנֵק מִשְּׁנֵיהֶם, וְהוּא עֵץ טוֹב וָרָע, וְקַיִן הוּא הַזֻּהֲמָה שֶׁהַנָּחָשׁ הִטִּיל בְּחַוָּה, וְעָלָיו נֶאֱמַר כִּי מִשֹּׁרֶשׁ נָחָשׁ יֵצֵא צֶפַע. מַה דַּרְכּוֹ שֶׁל הַנָּחָשׁ לַהֲרֹג, כָּךְ זֶה הָרַג וְאַחַר כָּךְ הִתְחָרֵט, וּמִשּׁוּם זֶה וַיָּשֶׂם יהו”ה לְקַיִן אוֹת, אוֹת בְּרִית מִילָה, וּמִשּׁוּם זֶה אָמַר קָנִיתִי אִישׁ אֶת יהו”ה, וְאַף עַל פִּי שֶׁקַּיִ”ן הוּא זֻהֲמָה, יָכוֹל לֵעָשׂוֹת נָקִ”י, כְּמוֹ יִתְרוֹ שֶׁנֶּאֱמַר בּוֹ וּבְנֵי קֵינִי חֹתֵן מֹשֶׁה.

Tikkun 69 – 125

Cain, קין, came from the impurity that the Snake put in Eve. It was said in Isaiah 14:29
“כִּי-מִשֹּׁרֶשׁ נָחָשׁ יֵצֵא צֶפַע” “for out of the serpent’s root shall come forth a basilisk”

Cain killed his brother because he was born with the killing trait inherited from the Snake. The Zohar tells us that Cain showed regrets and because of that God gave him a circumcision.

Genesis 4:15
“וַיָּשֶׂם יְהוָה לְקַיִן אוֹת” “And YHVH set a sign for Cain”

It was done so it will be known that he is in the process of cleansing his bad actions. The letters of Cain’s name קין can be rearranged to spell נקי, ‘clean’.

Even though Cain came from impurity, he was able to cleanse himself. In his reincarnation as Jethro, he came to Moses, who was the soul of Abel, brought Moses’s wife Tziporah and converted to connect to the Tree of Life.

We are born into different lives with different circumstances. We are always given a way to correct ourselves no matter how bad the Tikkun is.

The process of circumcision is made as the first Tikkun a child experiences in his life. It connects him to the Tree of Life and gives him a greater ability to overcome the hardship of the Tikkun.

The sin with the Snake caused a disconnection from the Tree of Life and connected Adam and Eve to a physical body. The foreskin is a piece of flesh that covers the ‘crown’ of the penis, which is the aspect of Yessod, the Sefira and the channel of Light to the lower parts of creation.