In the Zohar's Idra Zutra (the mystical secrets delivered by Rabbi Shimon Bar Yochai at the event of his passing from the world on Lag Baomer), we find this mystical teaching.
האי עתיקא אשתכח בתלת רישין
(This Atika is existing in Three Heads)
This is an inner most secret of the esoteric teachings of the Merkava, and in it are the inner keys to the mysteries of Creation. The AriZal opened the gates to these mysteries, and in this particular section of the Zohar (the Idros, particularly the Idra Zutra) every statement is saturated with Inner Wisdom of the Creator B"H.
This statement is introducing to us to עתיקא קדישא (Atika Kadisha) - "The Holy Ancient", Or "The Holy Copy", as these names are explained in Chassidus Chabad. Atika (or Atik sometimes) is the "point" where Creator and Creation "meet". Beyond this point everything is Creation.
Even if many "levels" below Atik are considered "G-dliness" and hence are not "real" - "egocentric" creation, From the point of Atik and on all levels are in some order of cause-effect progression, even though some of these progressions are in a form of "something from nothing", only in the World of Beriyah (Creation) is there first an existence of ego centrism, which is the World of Higher Garden of Eden, where the Neshamot (Souls) and Malachim (Angels) bask in the Light of the Schinah (Divine Pressence), this being the revelation of the Lights and Vessels of the 10 Sefirot of Atzilut in the lower 3 worlds through the form provided by Malchut of Atzilut, known as the Shecinah through the Torah and Mitzovs of Israel.
Back to this level of Atik, this is the highest point of Creation, and the "lowest" point of the Creator. Of course since the Creator is עצמו"ת אי"ן סוף ב"ה (The Essence of Ein Sof - The Infinite), any "point" in the Essence of Infinity, is Infinity.
This is a mystical "point" that always exists in everything, and through it everything comes into existence. It is also through this point that existence comes out of its shell and back into its original intrinsic existence.
It should also be noted in here, that the AriZal reveals in Pri Etz Chaim that what we said above that all creation originated in Atika, is also referring to the "outer aspect" of Atik, also known in Kabbalah as the ז' תחתונות עתיק. But in the Days of Moshiach, creation will originate from פנימיות עתיק (Pnimyut Atik), which is also called the level of Moshiach.
Considering what we write before about the two aspects in Arika, we gain some sense to what Moshiach is all about, but this is not the place for it.
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In the Zohar, Atika Kadisha refers to different levels, depending on perspective. In general, It refers to Adam Kadmon. To understand the relationship between Adam Kadmon and the Partzufim of Atzilut, particular the primordial Partzufim of Atik and Arich Anpin, is a discussion in its own right. It is in fact a journey, considering these relationships in perspective to the 3 general worlds of Akudim, Nekudim, and Brudim (not to be confused with the "standard" 4 worlds discussed in Kabbalah, beginning with Atzilut and the Bottom Three, Beryiah, Yetzirah, and Asiyah.
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This level of Atika, the Zohar says, is revealed with Three heads, which are:
רישא דלא אתידע - "The Unknown First" - It is a dimension within the Infinite Ein Sof
מוחא סתימאה - "The Hidden Mind" - The "second" instance in Keter of Atziut
ארך-אנפין - "The Long Aspect" - The revealed aspect of Keter of Atzilut
The rest of the Partzufim of Atzilut emanate for this final level, same as all 10 Sefirot emanate from Keter of Atzilut.
Returning to the Zohar's teaching, Atika is revealed in these three levels. To better understand this dimension called "Atika Kadisha" as well as what it means for this dimension to be revealed, in the Book of Daniel, Moshiach is said to be "Found, Hidden, and Revealed".
Similarly, in his commentary on the above Zohar, the AriZal uses the same terms for the revelation of Atika - Found, Hidden, and Revealed.
You can say in Kabbalistic terms, that at Radla Atika is initially "found, the Mocha Stima it is garbed and hidden, only to be revealed through Arich Anpin.
Its is interesting that the coming of Moshiach is likened to the same process, and therefore by understanding this process in more detail we can also better comprehend the coming of Moshiach.