Relativism would say that whenever there is debate over the merits of atheism/ humanism vs. theism in any regard, the question of God's existence by definition is the central problem of the argument because it is the question that defines both positions of the argument. However you have already taken the position that God's existence is axiomatic and have thereby used one possible conclusion as the premise upon which you argue the nature of 'meaning'.
The same could be said of many circumstances and beliefs that we might defend as justified, however only to find that we accept the circumstance or belief as an apparent a priori truth. For example, I hold that the individual freedoms we experience in the United States (as well as other Western countries) is a very, very good belief and circumstance, and it should be preferred over totalitarian societies where such individual freedoms either are inconsistent or do not exist. Yet, I live in the United States so the existence of the freedoms that I treat as axiomatic to proper human rights are in fact also my conclusion in that kind of argument (i.e., humans should have individual freedoms).
At some point in the great issues that concern us, we cannot help but arguing a position that we in fact accept at some axiomatic level. This is fine as long as we avoid logical fallacies and that we establish strong arguments.
If my argument is to assume that God exists, and from that argue that meaning is only possible by God's existence, then I have made some fallacies in reasoning, which you only need to point out that my conclusions are based on certain invalid inferences.
From the premise that God exists a priori, there follows a (theo-centric) presumption that some absolute and Platonic notion of 'meaning' also exists independently of our experience, and this notion of meaning is related to God by virtue of its absoluteness.
That's not my argument, however.
However, the concept of 'meaning' requires some referential framework containing a subject to whom that meaning applies, and some other 'thing' that acts as the definitive predicate for 'meaning'. As such the validity of subject A's meaning cannot be judged within subject B's frame of reference.
By the term 'meaning' I am talking about a transformation process that takes things,events,thoughts,etc that we are not so familiar and maps those things,words,events,thoughts,etc into things we are familiar in some satisfactory manner. For example, if I see the word 'happy', the word has meaning if I recognize it as a whole visual and mental array of memories having to deal with being happy. If I see the word 'homer', the word may have no meaning to you unless I tell you that it means a fan who thinks more of their sports team than reality would suggest as prudent. The term 'homer' now has meaning because I have related it to you be a whole series of words that somehow depict a satisfactory meaning to you. If I didn't explain it well enough, you might understand the meaning, but it wouldn't be satisfactory to you, and probably not usable in a sentence. In the case of afterlife issues, if I am not familiar with death, then for death to have meaning it should somehow map into my life's experiences. We can do so by saying that it is an unconscious state, etc, but humans are generally not satisfied with an endless unconscious state of non-existence in which we'll never see our loved ones we knew so well again. Such thoughts bring loneliness, sadness, great grief, etc. Saying death means an endless unconscious state might have meaning, but it is not a satisfactory enough to give real meaning to human existence beyond only a few 'momentary' pleasures.
However, the concept of 'meaning' requires some referential framework containing a subject to whom that meaning applies, and some other 'thing' that acts as the definitive predicate for 'meaning'.
So, using this understanding of the term 'meaning', the 'subject' is that which needs to have meaning attached (e.g., the word homer, or the concept of death, etc), the predicate is the mapping to something familiar (e.g., "is someone who overestimates their home sports team", or "is something similar to our life here and now, but where there is no unhappiness", or "is an unconscious state where we never see our loved ones again - sorry Charlie", etc).
As such the validity of subject A's meaning cannot be judged within subject B's frame of reference.
Sure it can, you just have to make sure that it is done so in a satisfactory manner. In the case of the word 'homer', I have to make sure that the way you seem to understand the word (e.g., how you use it in a sentence) jives satisfactory with the way I understand it. In the case of the meaning attributed to human existence by a non-belief in an afterlife, it is necessary that I see if for you an endless unconscious state is actually satisfactory to you (and all those who believe this or who potentially could believe this about what follows our natural lives). If it is apparent that most humans are not satisfied with it, then the concept of an endless unconscious state is meaningless for most humans. In those cases, you have to ask why a majority of humans would allow themselves to hold beliefs that made much of their thoughts about death to lead to a feeling of meaningless (i.e., unsatisfactory acceptance of the belief).
In other words 'meaningfulness' is logically relative and not absolute; and specifically, it's unfair for theists to establish a definition of 'meaning' as the pursuit of some purpose associated with man's relationship to God, and then proceed to chastise secularists for having a worldview that is apparently 'devoid of meaning'.
I agree it is relative, but it is relative to human satisfaction, not necessarily one's beliefs about what to hold a priori. This is why I can love freedom and hold it as a priori better than totalitarianism without ever having to hold the beliefs of totalitarianism. It is because I can hold that for most humans, their satisfaction is tremendously reduced in a totalitarian state versus a free society. As a theist, I can chastise secularists for allowing meaninglessness to infect them as well as the crowds they harm in the manner not too different than how I can criticize a totalitarian state for suppressing individual freedoms of their citizens.
The reductio ad absurdum to your claim to Mario is simply this: If an individual happens to believe that 'meaning' is (say)_to experience as much sybaritic hedonism as possible before one dies, then this individual could accuse a devoutly religious person of living a completely meaningless life.
Similarly, a totalitarian fan might say that experiencing government repression is freedom from individual rights and all the unnecessary choices that this presents and how it overly complicates our lives. Of course, it says nothing of overall satisfaction. Yes, it's true that it is nice once in a while not to have to make so many choices in a free society, but the satisfaction lost is nothing compared to the satisfaction gained in being able to make all of those choices. Similarly, satisfaction gained from believing in God and an everlasting life beyond this one is tons more satisfactory to the human spirit. If it were not the case, then you'd have to come up with a reason why human evolution has favored religion and not atheism. Yes, there are some satisfactory merits of atheism (e.g., you can live life the way you choose without having to look over your shoulder), but if this were indeed more satisfactory, then humans would have chosen this path a long time ago. The fact that they haven't chosen this path, I think, speaks volumes of the unsatisfactory nature of atheism, and even to a certain degree of secular humanism. Humans crave meaning in their lives, and since we are beings of beliefs, we tend to select the beliefs which provide meaning and reject the beliefs which don't.
Whereas tolerance for alternative notions of 'meaningfulness' demands some degree of this sort of relativism, one's capacity for such tolerance is restricted by any absolutism that assumes an exclusive definition of meaningfulness.
Tolerance is of course extremely important, but the tolerance I refer is allowing people to have the freedom to choose their religious beliefs without forcing them to choose otherwise. This doesn't mean that you say nothing in the hope of being tolerant. Tolerance is what - at the end of the day - that we all agree to be to each other, but that doesn't mean that we can't spout the importance of meaning and rejecting meaninglessness in the many forms by which it spreads itself. There are positive effects of temporary meaning, but the longterm trend can be meaninglessness and can create ill-desired effects for society as a whole. Drug addiction, for example, holds some temporary meaning (i.e., satisfaction to the drugs and experiences as they occur), but after the effects of the drugs the unsatisfactory experiences that follow can create many ill-desired effects to the individual and to those around them.
I think that a certain degree of this sort of absolutism that is built into Judeo-Christian/ Islamic thinking. (For example, see posts by JBrooks). All of these three monotheistic religions are deterministic-- which demands absolutism. I think that this is partly why the idea of 'secular meaninglessness' appears to be so obvious to most theists. It is exacerbated by their difficulty to imagine what their own lives would be like without worshipping God, which may lead them to assume that secularists must have replaced God with some non-theistic substitute: hedonism, humanity, money, etc. I think that the confusion arises because of the theist's need to impose the same spirituality that he or she experiences onto non-theists.
Religion, as wonderful as it is in providing humans with a meaningful existence, is also susceptible to shortterm effects that come about through living a meaningless endeavor. Priests that engage in sexual abuse, or ministers who use power to enslave members, etc, are all examples of how religion is not immune to the shortterm effects and longterm damaging factors of carrying on with some form of meaninglessness in one's religious activities. Balance is a key attribute in finding a spiritual center. This, by the way, is Jesus' major innovation to the Hebrew descendents of the time, and perhaps a major reason why so many were appealed to the Christian message. Jesus apparently taught basic freedoms, not only individual freedoms that changed how people should treat others, but also how religious heads should treat their members. Sometimes fundamentalists get more caught up in suppressing the needs of others than they are in finding real meaning in human existence and promoting that meaning to others. True religion is finding meaning in life that matters and not overindulging in the process so that life loses meaning with all the rules and requirements of the religion.
The irony is many secularists/ humanists/ atheists/ agnostics do care profoundly about the world (and their fellow man) —- sometimes to a greater degree than their theistic counterparts. Because of their belief in their own absolute mortality and the existential finiteness of their time in the world, secularists may see an ultimate preciousness in the world (and life) that might be greater than that of a devout theist-- who may see the world as only a stepping-stone to the ultimate preciousness of a promised afterlife.