This is an existantial argument from functionalism.
If you believe the basic tennets of theology, that life after death is the reward of good people, then we will live in a good world.
Many theologies do not hold that life after death is a basic tenant, I can think of at least one in which the aim of following the theology is to achieve a kind of eternal oblivion (Buddhism). Some might strike many people as very immoral and violent (Valhalla and the war of Ragnarok in Norse mythology).
Even amongst those that do believe in life after death the hope of a holy reward can be a motivator for good acts and evil ones.
I am sure that many members of organisations involved in inquisitions, witch hunts and holy wars thought that they were genuinely serving God and ensuring the salvation of souls - despite the fact that their actions caused great suffering.
Even some of the more intractable problems in the world today could be cast as the result of people who sincerely believe they are serving God - Zionist Settlers in the Occupied Territories and the Palastinian Islamic Militants who oppose them, the various actors in the War on Terror, and so on.
This isn't to say that secular thinking hasn't caused similar suffering or that all those involved in conflicts are sincere in their beliefs, but the evidence seems to suggest that belief in an afterlife isn't synonymous with good behaviour, and can lead to some very poor behaviour.
If you accept that your death is the end of you, then you have no reason at all to not destroy everything in a crass attempt to live just a little bit longer.
But your death does not mean the end of everything you care about or stand for. Many athiests have strong beliefs or bonds that have little to do with religion.
For a (highly negative) example I would once again mention the Tamil Tigers, who were willing to lay down their lives (in the act of killing others regrettably) in the hope of bringing about a socialist utopia - presumably so that their relations, friends and relatives could enjoy a "better world".
If a disbeliever in an afterlife (which is not the same thing as an athiest as far as I understand it) were to only behave in a selfish manner how does one explain such a phenomenon?
Also, what about athiests who succumb to ennui or despair? Why would they destroy just to live a little longer?
Seeing as though, very roughly speaking, our world (mostly) does advance, and our lives do improve, due to the goodness of others, one can only conclude that Creationism is better than Atheism, regardless of which is true or not.
Creationism is not synonymous with belief in an afterlife. Many monotheists exist who believe in an afterlife, but who take Genesis stories with a pinch of salt. I know of a great many people who are fundamentally reconciled to Darwinian ideas about the creation of the earth and the origin of life, but who feel that a God was the prime mover behind such acts.
I would not say the world has "advanced" as such, twentieth century history has seen far more destructive wars, genocides and periods of slavery than any previous century. Our management of our environment is so appalling that we risk destroying our civilisation. Scientific advances do not promise to bring people back from the dead (given debates on nature vs nurture, would a clone of Hitler even be Hitler - he was a product of his environment as much as of his genes surely?)
And seeing as though creationism is functional, whereas atheism is dysfunctional, we can only conclude that Creationism is THE TRUTH.
I don't see that you have made an argument in favour of creationism, but only in support of the idea that having a belief in some kind of eternal reward encourages people to behave well.
Also you don't define what creationism is for you
. Young Earth creationism? Intelligent design? A sort of deism that can comfortably accomodate Darwinian thought? Strictly as described in Genesis? Creation as understood by the Hindus? The Vikings?
Seeing as many athiests also seek a sort of purpose to their lives, and might like to think that they have "left the world a better place" I can't see how your argument concludes that belief in an afterlife leads to good behaviour and that therefore creationism is an apparent truth.
Would love to hear any further thoughts on the subject though.