In defending the truth of the Christian worldview, I often come across atheists who point to the supposed incompatibility of a loving God with the existence of evil and suffering. Many defenders of Christianity will move directly to explain to the atheist that without an eternal, unchanging standard, there is no objective morality, thus there is no objective good or evil. Without objective good or evil, their challenge is groundless. I agree with this answer, but only if the atheist is critiquing the Christian worldview from outside the Christian worldview; they are rather usually pointing to an internal inconsistency- that of a loving God and evil. Christians usually offer two answers to show that evil is, in fact, compatible with a loving God: that God does have purposes for allowing the evil, and man is free is disobey God which results in much evil and suffering. (Many do recognize that the challenge to Christianity has been overcome, but it is still offered in one form or another which does have much emotional and rhetorical power- more on this later.) However, this is only a portion of what our answer should be. We have merely shown that their claim of incompatibility is false, but what about challenges with atheism posed by evil and suffering?
The Assumption Granted
That question, of course, assumes the existence of an objective morality in the atheistic worldview. Many atheists do hold to some sort of "objective" morality. Most of the time this comes from the philosophical mistake of equating a teleological "good" with a moral "good". But I am not going to discuss that distinction in this post. For the purposes of continuing the conversation (and demonstrating another challenge to the atheist worldview), I will grant the idea that the atheist can have objective good in their worldview.
The Implications of the Assumption
As mentioned in the first paragraph one of the responses Christians give to the problem of evil and suffering is that God does have greater purposes for allowing it. The atheist often claims that evil is "gratuitous" because it has no purpose. The atheist is claiming to know ALL of God's intended ends (purposes) when they claim that "there is no purpose for <such-and-such> evil". This critique assumes something that the atheist worldview does not possess- a purposer. If evil and suffering do not have a purpose, then they are gratuitous- so the atheist claims. I ask the atheist who holds to the existence of objective morality, "where is the purpose for any of the evil and any of the suffering in your worldview?" Atheism by definition has no purposer, thus no evil or suffering can have a purpose. Thus all evil and all suffering is gratuitous.
Further Implications of the Assumption
As many people who engage in the debate over God's existence are aware, one of the arguments put forth in support of God's existence is the teleological argument. One version offers that the universe has values of the laws of physics that are hundreds of orders of magnitude (exponentially) higher than the precision of any man-made object; since less precise objects are intelligently designed, more precise objects must be the work of an even greater intellect.
Quite a few atheists have offered that a multitude of universes exists with different values for the laws of physics. It is often posited that an infinite number is necessary for a random "universe generator" to produce one with the values of our universe. Without going into the issues of an actual infinite number of objects (notice again that I'm granting a point that I don't necessarily need to), an infinite number of universes means that not only do you get an infinite number of universes with different values for the laws of physics, but you also get an infinite number of universes with the same laws of physics.
This is extremely important when we consider the problem of evil and suffering- especially from the atheist's worldview. If there is an infinite number of universes with the same laws of physics as ours and that produces humans, we now have an infinite number of universes with evil and suffering. Christianity has a finite number of universes (one that we're certain of- see Who's Afraid of the Multiverse? by astrophysicist Jeff Zweerink) with a finite amount of evil, that will come to an end. Evil in the atheist's multiverse exists in infinite amounts, has no purpose and will never come to an end.
Application of the Implications of the Assumption
Now, unlike the challenge to Christianity, I am not offering this as an incompatibility (there is no loving purposer in an atheistic worldview for evil and suffering to be incompatible with). I am offering it as a necessary consequence- one that the atheist must be able to deal with. One in which they have no answer to when a person who is suffering the death of a child to cancer comes to them for comfort. One which brings even more unnecessary suffering and pain to every individual seeking answers and comfort because they have no answers.
An Emotional Argument?
The existence of evil and suffering cuts directly to the heart of every person (whether objective evil exists or not). It is extremely emotional. So isn't this an emotional argument? Absolutely. Can it be dismissed based on that fact? Absolutely not! Whether the conclusion has emotional force or not is irrelevant. The conclusions here have been derived from reason and science. If the atheist wishes their audience to be dissatisfied about a finite amount evil and suffering with ultimate purpose in reality that will end, why should they not expect their audience to be dissatisfied with an infinite amount of evil and suffering without ultimate purpose in reality and will never end? These are logical questions that arise from careful reasoning with what scientists offer us as the truth of reality.
Conclusion
The problem of evil and suffering is a bigger problem than the atheist thinks. Not only is it not a problem for Christianity (for several reasons), but it poses insurmountable issues with reason and emotion- a cognitive dissonance that only someone committed not to the truth, but to a false worldview, could possibly tolerate.