My questions is: since atheists don't believe in an Afterlife where any Truth will be exposed...
do they strive for it on Earth, at least?
Some atheists do believe in such an afterlife. I do not, but I do not see this as impeding my ability to strive for truth.
The pursuit of pleasure is the purpose of life?
This is a common position among atheists. I disagree, but I understand it. We evolved to survive and reproduce and the reward mechanism we developed to motivate us in that process is the pleasure that comes from satisfying our base instincts, such as the desire for sleep, food, water, safety, and sex.
As social creatures, we evolved to cooperate to meet our goals. Since our goals are usually derived from an emotional attachment to the aforementioned sensory pleasures, this means that ancient human cultures developed social mores and enforced rules which were meant to help achieve these pleasures. In that way, we can say that morality is based on pleasure.
This is a description of common morality that we can find in anthropology and social psychology. There is the additional claim that there is no difference between describing what is moral and asserting what one "ought" to do, since morality implies some degree of normativity. That position is known as moral naturalism and that quite neatly leads to hedonist ethics, such as Epicureanism or Hedonic Utilitarianism, when we account for the above factors.
This is not quite the depravity that a religiously-minded person might think, though. A Hedonic Utilitarian would sacrifice their own life to save two other people, for example, and an Epicurean would be inclined towards practicing asceticism. That might seem paradoxical, but these philosophies have well thought-out reasons for this.
In my opinion, I think the entire argument commits the Naturalistic Fallacy. Instead, I agree with Socrates that moral virtue comes from knowledge, and I think the Stoics were on the right track when they developed a system of virtue ethics geared towards living a life in accordance with logic in order to pursue knowledge efficiently.
I honestly see religious ethics as fallacious, too. Even if God exists and has provided us with moral law, asserting that we should follow that moral law would be an Argument from Authority, which is an informal fallacy. In my opinion, there exists no rational apparatus that could derive that one "ought" to follow these laws simply from a description of what the morality within them "is." To me, this is almost identical to the error in logic that moral hedonists make, because both attempt to derive an explicit "ought" from a description of how one particular moral framework "is."
The only reason that truth stands unmarred as an objective normative standard is because we implicitly affirm its value whenever we make an argument. Logic deals with truth, and inherent to logic is a normative goal for achieving truth. Any time we make an argument for any ethical standard, whether it is Utilitarianism or Divine Command Theory, we are using a method (logic) whose aim is to achieve truth. So every ethical position, including moral nihilism, starts with truth as a value.
The only problem is that they tend to assert other values and place them as a higher priority than truth without adequate justification. It is not that truth is necessarily the only moral value, but, as far as I am aware, no adequate argument for a value other than truth has ever been made. I think Hume's Guillotine makes such an argument infeasible to construct.
I am not an atheist because I inherently value atheism or have a moral stake in its truth. I'm only an atheist because I think we have good reason to believe that the universe is not created (and is probably best conceptualized as an eternal 4D "block" of spacetime) and that minds are emergent processes of physical matter, which I arrive at through inductive reasoning. I consider it my moral obligation to admit these facts as they occur to me, because they are the most likely given the information that I have available to me. It would be my obligation to relieve myself of them in the event that they are shown to be likely to be false or less likely to be true than a superior conclusion.
By contrast, my moral sentiments are deductively true, if not outright tautological, and therefore I can say that I am absolutely certain of them. There might be some error in my line of reasoning or a strong/valid argument for some other moral value that I have yet to see, and I am open to that, but I think it would be an uphill battle to substantiate.
