In my college days, I owned a shirt that read, in huge bold letters, "YOU NEED JESUS BRO. I'M JUST SAYIN'." One of my classmates, though an atheist herself, loved that shirt, and pointed it out to the rest of the class. To this, another of my classmates asked "what if you're an atheist?" In hindsight, the answer I wish I'd given is "well that won't make Hell a degree colder", but unfortunately, my sheepish college brain was taken off guard, and all I could say was "you still need Him".
I don't remember how the rest of that conversation went, but it does illustrate a very common phenomenon with atheists. In fact, arguably, it forms the basis of atheism itself: A lack of perception is often confused with a lack of need. Atheists at least claim to not believe in God, and thus when their needs are fulfilled, they reason that they don't need Him. They feel they don't need Him for their Earthly lives, since they fulfil their own needs here, and they don't need Him for their spiritual lives, since if they believe in a spiritual realm at all, they find their fulfilment elsewhere too. A variant of this attitude is also as a response to the moral argument; "I don't need God to be a good person".
First, let us address what the moral argument really is. Atheists often misinterpret what Christians are saying. In their mind, we're saying if you don't believe in God, you will necessarily become pure evil, and you have no reason not to murder, rape, steal etc. In reality, however, the argument isn't that Theism makes people good and atheism makes people evil. In fact, a careful study of scripture shows that atheists, and people in general, will be a mixed bag of good and evil.
Romans 2 is an excellent example of exactly this. Here, Paul not only discusses the purpose of the law, but in verses 14-15, even shows that unbelievers, who do not have the scriptures, instinctively do what is in the scriptures, and that this is precisely why they can be condemned for sin in spite of not knowing the scriptures. So, Biblically speaking, yes, you can be "good without God". The problem is, without some form of God, what we think of as "good" and "evil" are really just that: what we think of as good and evil.
But even among atheists, opinions differ. Some atheists are vegans and regard the consumption of eggs as being comparable to slavery, others think human beings are the absolute centre of morality, and even animal cruelty is an acceptable means to that end. Some atheists are charitable, even altruistic, whereas others are very selfish and stingy. Some atheists value equality, whereas others start entire regimes based on the premise that their groups are superior. The question to ask any atheist is "what makes your opinion better than other atheists?" And they'll never be able to answer.
This is because good and evil are inherently religious concepts. Just as there must be a governing body in order to call something "illegal", there must also be a God to call something "evil". Otherwise, "morally wrong" is really just "morally different". So basically, while atheists may not need God to act according to their own moral views, they do need Him to justify those views.
But do they really not need Him to form those moral views? When we dig deep, we find that actually, "I don't need God" is a thoughtless and inaccurate statement based on arrogance. To illustrate this, consider the old joke:
"Dear hunters,
You ought to be ashamed of yourselves. You don't need to go out killing things to eat. You should just get your meat from the supermarket, where no animals were harmed."
This childlike mentality is funny, because any adult understands the serious flaw in that reasoning. Yes, you can buy meat from the supermarket, but where does it come from before it gets to the supermarket? Yes, an animal had to die. That's what meat is. It's the severely mutilated carcass of a deceased animal that may have even had a particularly stressful death.
When it comes to our need of God, we tend to think no further back than the "supermarket" phase. But no Christian believes God is a puppet master controlling every flick of the finger, nor that He just sends food straight into our mouths like someone spoon feeding a child. He certainly blesses obedience, but there is an entire book of the Bible (Proverbs) dedicated to telling us that the world God has created operates on a system of cause and effect that will work regardless of one's personal faith.
In fact, throughout the Bible, we see that faith tends to be the more difficult route in the short run, and wickedness tends to breed temporary prosperity. Psalm 14, for example, addresses those who say in their heart "there is no God", who prosper off the very backs of God's people. Like a bully who profits off of stealing lunch money, never once believing there are authorities who will eventually deal with him, unbelievers of all kinds may well prosper precisely by rebelling against God. But this prosperity is both temporary and illusory. It is temporary, as death itself is inevitable, and not a penny will follow a man to the judgement seat of God. It is illusory because there is a God.
This God, according to one of the most famous portions of scripture, "...makes His sun rise on the evil and on the good, and sends rain on the just and on the unjust." (Matthew 5:45). It even cites this as a standard for us, as believers, to love even our enemies, in order to be God's children. Atheists, therefore, receive God's blessings by default.
Ultimately, God provides for our needs in ways we don't typically consider. As our heart beats without us even thinking about it, God provides for us without us perceiving it. But one thing atheists may deny, but do so against all reason, is that God designed that beating heart. All things in Heaven, and on Earth, were created by Him, and continue to be sustained by Him. That includes the ground upon which atheists walk, the food and water with which they prolong their lives, even the very breath with which they blaspheme Him. And going back to the earlier moral discussion, yes, even their natural moral inclination to do good comes from the God in whose image they were created.
Not even unbelief is enough to snuff this out. Knowledge of truth is often distinct from the existence thereof. Atheists would actually have to admit this, as they tend to feign a commitment to science (while simultaneously hijacking it in order to give intellectual basis to their atheistic delusions). Science is an ever-changing field, as not only are new truths discovered, but also, what was once rejected is vindicated, and what was once firmly held is disproven. Yet, truth is unchanging. Gravity, for example, existed before we discovered it.
And then we learned to fly. Science is actually an example of knowledge we don't necessarily need, but having that knowledge is beneficial to us. We can live long and happy lives with minimal scientific understanding, but once we understand the world, we can make better use of it. By contrast, a poor understanding of science fails to yield these benefits, and may even result in some very serious problems.
In much the same way, purely by virtue of the fact God's world is designed to be inhabited by humans in general, not just believers, it is entirely possible for us to live long and happy lives without directly acknowledging Him. However, obedience to God's word tends to yield better results for everyone, whereas disobedience generally leads to multiple parties being in some way injured. Many atheists will even acknowledge this when posed with the question of whether they think the world would be better or worse if everyone in it was a Christian vs. if everyone in it was an atheist?
So we see that atheists, whether they perceive it or not, do need God, and that He, in His grace, actually fulfils those needs. But we all have a greater need of God, as while this world is important, it is also temporary. Eventually, every single one of us is going to die, at which point we will be judged for every deed, word, even thought. But when the Holy God is the judge, every hint of sin counts. If the standard is perfection, and indeed it is, then imperfect people will inevitably fall short. And every single one of us does.
The wages of sin is death. We, who have sinned against God, are, in His omniscient eyes, worthy of wrath. However, God is not one dimensional, like a fire burning wood. Rather, just as He is perfectly just, He is also perfectly loving. For this reason, scripture tells us He takes no pleasure in the death of the wicked, and does not want them to perish. Rather, He wants all men to be saved and come to the knowledge of the truth. Salvation, however, has its own requirements.
Sin must be dealt with, or God cannot claim to be just. But who pays that debt is open for discussion. The simplest answer would be you did the crime, you do the time. But another option is Jesus, who did not do the crime, can do the time on your behalf. Which He willingly did. Scripture tells us that Christ, who knew no sin, became sin for us, that we might become the righteousness of God in Him. In effect, we "swap verdicts". So, the biggest way in which atheists need God is that they have sins for which they cannot atone on their own.
So, why do atheists need God? For three reasons. First, they need Him to fulfil needs He continuously fulfils without them even perceiving. He constantly provides for them by sustaining His creation. Second, they need Him to improve their life on Earth, not simply increasing their pleasure, but actually working towards a brighter future for themselves as individuals, and for the Earth as a whole, which would undoubtedly prosper if all people obeyed God. Finally, they need Him to save them from an eternity of justice, paying the due penalty for their sin. No "lack of faith" can spare them an eternity of doom, but faith will guarantee them a wonderful eternity.