First, I have already referred to the following question - Does Bhagavad Gita detest atheists? But that didn't help me.
I have been reading a translation of the Gita since a few days; I came across a line saying that those who deny my existence are foolish and demonic. So my question is: Does that say that every atheist is bad, or only the desireful, and the ones that treat other people badly?
Bhagavadgita condemns only atheists who do evil acts to destroy the world. This is what Gita says about Demonic people -
Those who are demoniac do not know what is to be done and what is not to be done. Neither cleanliness nor proper behavior nor truth is found in them. They say that this world is unreal, with no foundation, no God in control. They say it is produced of sex desire and has no cause other than lust.Following such conclusions, the demoniac, who are lost to themselves and who have no intelligence, engage in unbeneficial, horrible works meant to destroy the world.Taking shelter of insatiable lust and absorbed in the conceit of pride and false prestige, the demoniac, thus illusioned, are always sworn to unclean work, attracted by the impermanent. They believe that to gratify the senses is the prime necessity of human civilization. Thus until the end of life their anxiety is immeasurable. Bound by a network of hundreds of thousands of desires and absorbed in lust and anger, they secure money by illegal means for sense gratification.The demoniac person thinks: “So much wealth do I have today, and I will gain more according to my schemes. So much is mine now, and it will increase in the future, more and more. He is my enemy, and I have killed him, and my other enemies will also be killed. I am the lord of everything. I am the enjoyer. I am perfect, powerful and happy. I am the richest man, surrounded by aristocratic relatives. There is none so powerful and happy as I am. I shall perform sacrifices, I shall give some charity, and thus I shall rejoice.” In this way, such persons are deluded by ignorance. Thus perplexed by various anxieties and bound by a network of illusions, they become too strongly attached to sense enjoyment and fall down into hell. Self-complacent and always impudent, deluded by wealth and false prestige, they sometimes proudly perform sacrifices in name only, without following any rules or regulations. Bewildered by false ego, strength, pride, lust and anger, the demons become envious of the Supreme Personality of Godhead, who is situated in their own bodies and in the bodies of others, and blaspheme against the real religion. (BG As It Is 16.5-18)
Gita acknowledges the right of a person to reject its teachings.
‘..Reflecting on this entire teaching, do as you think fit.’
Gita 18.63
However, Gita has very low opinion about atheists.
According to them nothing is ultimately real in this world. It is Godless and without any moral basis. Being born of sex union, what else but lust can be said to be its cause?
Gita 16.8
We need to deal with 2 cases, that of immoral atheists and moral atheists.
immoral atheists
What happens to immoral atheists?
These vicious men, oppressive, cruel and sinful as they are, - are always hurled down by Me into demoniac wombs in life after life, in the transmigratory cycle.
Gita 16.19
What Gita means is that immoral atheists would remain in Samsara, the cycle of birth and death, since they will not do spiritual practice to purify their hearts and know God and will thus suffer.
Moral Atheists
Of course there are also atheists who live moral lives. What would happen to them? The answer is that they will also remain trapped in Samsara since God realization is the only way out of Samsara. However, they will most probably have pleasant life since they have lived a moral life.
The only way to end of sorrow is to know God. When men shall roll up space as if it were a piece of leather, then will there be an end of sorrow, apart from knowing God.
Svetasvatara Upanishad VI.20
Does Gita discriminate between atheists and believers?
The answer is no. Even believers will remain trapped in Samsara unless they purify their mind.
For the attainment of mental purity, spiritual aspirants (Yogins) perform action devoid of attachment, with their body, mind, intellect or even merely with the senses.
Gita 5.11
Intellectual position irrelevant for moksha
The point is that Gita or Hindu shastra does not say that intellectual belief or disbelief plays any role in the fate of a person. It is only mental purity that determines whether a person will be able to escape the cycle of Samsara. An atheist will remain trapped in Samsara not because of his belief but because of not striving for mental purity. It is his choice. Similarly a believer who does not purify his mind will also remain trapped in Samsara. Again it is his choice. God does not play any role in deciding whether any person remains in Samsara or not.
I think you are talking about this verse.
B.G.9:11.Fools deride Me when I descend in the human form. They do not know My transcendental nature as the Supreme Lord of all that be.
This verse is not talking about those who don't believe in god but about those who don't believe in that god posses a form. Here, Lord Krishna is telling Arjuna that even though the parambrahman is formless (Yajurveda 32:2) he can take human form. The Supreme Lord has created this entire world full of forms, shapes, and colors. If He can do such an amazing feat of creating myriad forms in the world, can He not create a form for Himself? Even Upanishads hold the same opinion.
Shwetashwara Upanishad 4:1 May that Divine Being, who, though Himself formless, takes various forms in different ways with the help of His own power, for His own inscrutable purpose, and who dissolves the whole world in Himself in the end - may He endow us with good thoughts!
About atheism, Lord Krishna has given the following statement;
BG 5.4 - The fools, not the learned ones, speak of Sankhya (the path of Knowledge) and (Karma-) yoga as different. Any one who properly resorts to even one (of them) gets the result of both.
Sankhya is the atheist philosophy of hinduism and lord Krishna clearly said that they can attain moksha if they follow sankhya correctly.
