He said, "Believing in God and seeking mental power from him is a
symptom of a weak mind.
Is my mind really weak? Is that my tears, goosebumps and all the
feelings, just my sentiments?? please answer. I really need
Atheists are typically the type of people who don't believe in the existence of external gods. Given, that he doesn't believe in any existence of gods in general, it is very easy to establish that you aren't seeking any mental power from any external agent. In such cases when no external influence affects the mind and strengthens it, then, it is only your mind that is the source of all the bliss and happiness, hence not weak (contradictory to his statement).
Is it possible to prove the existence of external God?
No. It isn't.
This is why there are externally godless versions of philosophies that come into play that don't require any external gods by default.
In this view, jivatman, the experiencing self, is ultimately
non-different ("na aparah") from Ātman-Brahman, the highest Self or
Reality.[3][7][8][note 4] The jivatman or individual self is a mere
reflection or limitation of singular Ātman in a multitude of apparent
individual bodies.[9]
Wikipedia. Advaita Vedanta. Date Accessed: 15-01-24
Sankhya/Yoga School:
According to Sinha, the following arguments were given by Samkhya
philosophers against the idea of an eternal, self-caused, creator
God:[153]
If the existence of karma is assumed, the proposition of God as a moral governor of the universe is unnecessary. For, if God enforces
the consequences of actions then he can do so without karma. If
however, he is assumed to be within the law of karma, then karma
itself would be the giver of consequences and there would be no need
of a God.
Even if karma is denied, God still cannot be the enforcer of consequences. Because the motives of an enforcer God would be either
egoistic or altruistic. Now, God's motives cannot be assumed to be
altruistic because an altruistic God would not create a world so full
of suffering. If his motives are assumed to be egoistic, then God must
be thought to have desire, as agency or authority cannot be
established in the absence of desire. However, assuming that God has
desire would contradict God's eternal freedom which necessitates no
compulsion in actions. Moreover, desire, according to Samkhya, is an
attribute of prakṛti and cannot be thought to grow in God. The
testimony of the Vedas, according to Samkhya, also confirms this
notion.
Despite arguments to the contrary, if God is still assumed to contain unfulfilled desires, this would cause him to suffer pain and
other similar human experiences. Such a worldly God would be no better
than Samkhya's notion of higher self.
Furthermore, there is no proof of the existence of God. He is not the object of perception, there exists no general proposition that can
prove him by inference and the testimony of the Vedas speak of prakṛti
as the origin of the world, not God.
Wikipedia. Samkhya. Date Accessed: 15-01-24
Nyaya School
As a matter of fact, we find that Man, desiring a certain thing, does
not always obtain the fruit of his desire; hence it is inferred that
Man's acquisition of the fruits of his actions is dependent upon some
other person; and that person upon whom it is dependent is God; hence
it follows that God is the Cause[.]... "If the appearance of fruits
were dependent on God, then such fruits could be accomplished even
without the desire of man." ... As a matter of fact, God helps the
effort of Man; i.e., when Man is trying to obtain a particular fruit,
it is God that accomplishes that fruit for him; when God does not
accomplish it, Man's action becomes fruitless; hence since things are
thus influenced by God, what has been urged to the effect that - "as a
matter of fact no fruit appears without Man's action" - is no reason
at all.
https://hinduism.stackexchange.com/questions/21975/how-do-different-astika-schools-prove-the-existence-of-god/21976#21976:~:text=As%20a%20matter%20of%20fact%2C%20we,action%22%20%2D%20is%20no%20reason%20at%20all.
Nyaya School (Rejection on the existence of the God)
The Lord is the cause, since we see that human action lacks results.
This is not so since, as a matter of fact, no result is accomplished
without human action. Since this is efficacious, the reason lacks
force.
— Nyaya Sutra, IV.1.19 - IV.1.21 [57]
A literal interpretation of the three verses suggests that Nyāya
school rejected the need for a God for the efficacy of human activity.
Since human action and results do not require assumption or need of
the existence of God, sutra IV.1.21 is seen as a criticism of the
"existence of God and theism postulate".[57]
Wikipedia.Nyaya. Date Accessed: 15-01-24
Definition of God, through Self
In summary, it is difficult to prove the existence of external god through any means to an atheist. There are means to get over it via the Advaitic Definition of God through self via Taittiriya Upanishad: https://www.youtube.com/watch?v=Ftn4zCnheBk