After Moksha, since we don't have a body, we have no Consciousness or Existence , then how can we conclude that Brahman exists? Because Consciousness and Existence cease to exist after death. I am not able to understand how can Consciousness exist after Moksha.
Consciousness is independent of the body. Brahman does not depend on the body. All appears on consciousness.
Talk 396, Talks with Sri Ramana Maharshi
Life is existence which is your Self. That is life Eternal. Otherwise can you imagine a time when you are not?
Where or how were you before being born? Were you in sleep? How were you? You exist then too without the body. Then the ego arises, and then the mind which projects the body. ‘I-am-the-body’ idea is the result. Because the body exists you say that it was born and that it will die, and transfer the idea to the Self saying that you are born and that you will die. In fact you remain without the body in sleep; but now you remain with the body. The Self can remain without the body, but the body cannot exist apart from the Self.
Births, being of the body,cannot affect the Self. The Self remains over even after the body perishes. The discontent is due to the wrong identity of the Eternal Self with the perishable body. The body is a necessary adjunct of the ego. If the ego is killed the eternal Self is revealed in all its glory.
Waking consciousness is that state of consciousness where it is in relation to physical objects. Dream is in relation to dream objects. In deep sleep, there are no objects but there is happiness but this is not the same as the bliss. Mandukya Upanishad tells us all these three states are phenomenal. These are states to be transcended, and the real nature of consciousness cannot be recognised or seen either in waking, dream or sleep. Real Consciousness is the “Fourth” (Turiya). It is the fourth only in number because it is beyond all three and is inclusive of all that is in the other three states of consciousness.
nāntaḥ-prajñam, na bahiṣ prajñam, nobhayataḥ-prajñam, na prajnañā-ghanam, na prajñam, nāprajñam, adṛṣtam, avyavahārayam, agrāhyam, alakṣaṇam, acintyam, avyapadeśyam, ekātma-pratyaya-sāram, prapañcopaśamam, śāntam, śivam, advaitam, caturtham manyante, sa ātmā, sa vijñeyaḥ (Ma.U. 7).
In this state, what are we? Conscious. Conscious of what? Not of external things, and not of internal things. Nobody can say what it is…We are Atma, We are That. We are the universality of consciousness [wherein the differentiation between the subject and object disappears]
I think this touches the questions answer upto some extent. From Sarvollāsatantram 63rd Ullahasa
मृत्तिकायां घटो देवी हाटके कटकं यथा । शुत्तौ रजतवत्तोये तरङ्गबुद्बुदौ यथा ॥ रज्जौ भुजङ्गबेद्देवि स्फटिके रञ्जनं यथा । तथा देहो महेशानि देहिमध्ये प्रकाशित: ।।
The way pot (ghat) is murti, the way neckless is of gold, the way bubbles form in water, the way there is illusion of snake in roap, the way color hides the quartz, the same way there is a 'Dehi' in 'Deha'!
घटे नाशे मृत्तिका स्यात् कटके हाटकं तथा । तरङ्ग-बुदबुदौ नावों तोयमात्रं न संशय: ।।
No doubt, Breaking the Pot leads to just soil, melting the neckless leads to just gold, as the bubbles pop only the water remains.
भुजङ्ग-नाशे रज्जुः स्यात् रज्जने स्फाटिकं तथा । देहनाशे तथा देही केवलोऽहं निरज्जनः॥
By removal of the illusion of snake leaves just the rope, the removal of color leaves just the quartz, the same way the death of the body leaves just the 'Dehi' which is of form of an eternal void. (Nitya Niranjan).
These verses are ofcourse talking about the Atman that resides within the body, and thus it realizes upon that Brahman. (Signified with the Atmans merging into ParmAtman!?). 'Dehi' actually means a thing that has wearied the 'Deha' (body).