Atman, verily Brahman, is Ever Established
In the Brahmas?tra bh??ya for the very first sutra, Shankara says:
?tmatv?t sarva–bh?t?n?m? siddhatv?d iha sarvatah?
na not; hi indeed; acyutam the infallible Supreme Personality of Godhead; pr?n?ayatah? satisfying; bahu much; ?y?sah? endeavor; asura–?tma–j?h? O sons of demons; ?tmatv?t because of being intimately related as the Supersoul; sarva–bh?t?n?m of all living entities; siddhatv?t because of being established; iha in this world; sarvatah? in all directions, in all times and from all angles of vision.
?r?mad Bh?gavatam 7.6.20-23par?vares?u bh?tes?u brahm?nta-sth?var?dis?u
bhautikes?u vik?res?u bh?tes?v atha mahatsu ca
gun?es?u gun?a–s?mye ca gun?a–vyatikare tath?
eka eva paro hy ?tm? bhagav?n ??varo vyayah?
pratyag?tma–svar?pen?a dr??ya–r?pen?a ca svayam
vy?pya–vy?paka-nirde?yo hy anirde?yo vikalpitah?
keval?nubhav?nanda-svar?pah? parame?varah?
m?yay?ntarhitai?varya ?yate gun?a–sargay?
para–avares?u in exalted or hellish conditions of life; bh?tes?u in the living beings; brahma–anta ending with Lord Brahm?; sth?vara–?dis?u beginning with the nonmoving forms of life, the trees and plants; bhautikes?u of the material elements; vik?res?u in the transformations; bh?tes?u in the five gross elements of material nature; atha moreover; mahatsu in the mahat–tattva, the total material energy; ca also; gun?es?u in the modes of material nature; gun?a–s?mye in an equilibrium of material qualities; ca and; gun?a–vyatikare in the uneven manifestation of the modes of material nature; tath? as well; ekah? one; eva only; parah? transcendental; hi indeed; ?tm? the original source; bhagav?n the Supreme Personality of Godhead; ??varah? the controller; avyayah? without deteriorating; pratyak inner; ?tma–svar?pen?a by His original constitutional position as the Supersoul; dr??ya–r?pen?a by His visible forms; ca also; svayam personally; vy?pya pervaded; vy?paka all-pervading; nirde?yah? to be described; hi certainly; anirde?yah? not to be described (because of fine, subtle existence); avikalpitah? without differentiation; kevala only; anubhava–?nanda–svar?pah? whose form is blissful and full of knowledge; parama–??varah? the Supreme Personality of Godhead, the supreme ruler; m?yay? by m?y?, the illusory energy; antarhita covered; ai?varyah? whose unlimited opulence; ?yate is mistaken as; gun?a–sargay? the interaction of the material modes of nature.
This particular expression in the above verse is also completely reflected in the Sh?nkaran bhashyas:
// pratyag?tma–svar?pen?a dr??ya–r?pen?a ca svayam //
That One Supreme Being alone is available as the Atman of all and everything that is experienced as d??ya, the seen. Therefore, the d?k and the d??ya are both the Self alone, implying the sarvam khalvidam brahma theme: (This) Atman is Brahman.// found in the above cited BSB.
This word of the bhagavatam cited above //siddhatv?d // too is significantly present in the Taittiriya bh??ya 1.11.4
????????????????????? [Those who have the realization of the ever-established Atman.]
The following Brahma sutra bhashya sentences too are about the siddha vastu that the Self, that is Brahman is:
BSB 1.1.2:
?????? ?????????????????????? ??????????? ????????, ???????????????? ??????????????????? ??????????????
Shruti, etc. and anubhava, etc. are pran??a here, as the context demands. This is because the quest for knowledge culminates in ones experience of the ever existing Brahman, which alone is the subject matter of that knowledge/experience.
BSB 3.4.52:
????? ??????? ??????????????????? ??????? ???????? ??????????????? ?
That (liberation) indeed is incapable of being produced since It is of the nature of being Ever Established.
Om Tat Sat