Holy Zohar text. Daily Zohar -379.

Tikkun 21 – 120

The righteous people are written in the Book of Life ספר החיים on Rosh Hashanah because they keep the connection to חי, life, which is the clinging to the Tree of Life. חי is numerically 18, which represents the 9 Sefirot of direct and 9 of returning light.
(Malchut doesn’t have light of its own)

Deuteronomy 4:4 “וְאַתֶּם, הַדְּבֵקִים, בַּיהוָה, אֱלֹהֵיכֶם–חַיִּים כֻּלְּכֶם, הַיּוֹם”
“And you that cleave to YHVH your God are alive every one of you this day”

For those that their actions are mixed with good and bad, the judgment is done on Yom Kippur. The ten days between Rosh Hashanah and Yom Kippur gives us the opportunity to make Teshuvah, repentance, to remove the negativity from our path.

Ezekiel 33:19 “וּבְשׁוּב רָשָׁע מֵרִשְׁעָתוֹ, וְעָשָׂה מִשְׁפָּט וּצְדָקָה–עֲלֵיהֶם, הוּא יִחְיֶה”
“And when the wicked turned from his wickedness, and does that which is lawful (following the spiritual laws and connect to the central column) and right (elevates light), he shall live.”

The Zohar relates Yom Kippur to Noah’s Ark. If we are worthy then we can be on the ‘Life saving’ ship and if not,
Genesis 7:21 “וַיִּגְוַע כָּל-בָּשָׂר…”
“And all flesh perished…”
As we are getting closer to the end of the Tikkun, we won’t have opportunities to come back to life to complete our correction. Every action we do now must be part of the correction process. There’s no rest on Friday when we’re working for ‘Shabbat’.
Our sages said that “אין מקבלים גרים לימות המשיח”
“No acceptance of converts in the days of the messiah”

When the Tikkun is completed then there’s no more work left to elevate that light.
No matter if you are a Jew or not. The conversion process is to accept and follow the pure system of YHVH, which is the Tree of Life.