I haven't been a practicing Christian for a long time, and am, as I often say, an Apathetisist, "God may or may not exist, but it won't affect me either way." Still, I am able to think from the perspective of one, or at least, from as much the perspective that I ever managed, which was always just outside of the mainstream anyway.
Creationists and Intelligent Design folks are Christians in name, but in reality, they have turned against God, hidden their faces from his might, and become like the Pharisees of the Bible. They have actively, willfully, and knowingly, placed themselves on a pedestal above God, and seek to dictate His doings according to their understanding.
First, let us do away, simply, with the need for a literal interpretation of the Bible. The Psalms are not literal, nor is the "Song of Solomon". We are told to treat the "Song" as an allegory of God's love for us. Knowing then, that there is a deeper truth behind some elements of the Bible, perhaps we shouldn't cling to closely to the idea of a literal interpretation. Especially when any biblical scholar knows that there is no perfect translation.
Second, let us accept that God did not just give us the Old Testament and the New Testament, but there is a third volume of His work that gives us insight into His nature, and that is the world around us.
Third, one thing that all of these volumes agree on is that God created Man and gave him the capacity for wisdom, discernment, and the ability to comprehend the world around him.
Fourth, given the preceding, we have learned certain facts about the world. For one, its age is far greater than can be achieved through a literal interpretation of the Bible. Living beings appeared in sequence, gradually developing into various forms, each suited to their environment; some died out, some were replaced by different forms, and some continue to this day. We can map the patterns of this using the tools of our reason, and the consistent physical laws of the Universe that God developed.
So far, I have not showed Creationists as the Blasphemers they are, but I have shown that their world-view has no backing, either theologically, or scientifically. Let me explain how they blaspheme.
A man walks into a room, and places a single domino upon the table, standing upright, and proceeds to knock it down. This isn't terribly exciting, is it? Would we give any respect to this man for that act?
Another man walks into the same room, and takes ten thousand dominoes, and builds patterns, and structures, passing them over bridges and across tabletops. He constructs miniature catapults to fling dominoes from one end of his masterpiece to another, and includes marbles and all variety of Rube Goldberg arrangements.
Which is more remarkable? If we wish to give all glory to God, if we wish to show true devotion to the Creator, which, of these two creators, would be more worthy of that praise? The one that simply created the world with a word? Or one that created a world that changes and develops, that is a dynamic system? We would expect that God would have created the world in the most glorious manner at his disposal, and this complexity of nature, this four billion year old Earth we inhabit, is proof that God is the second variety. Should we be surprised that God can master strange forces and bring about a world that develops without the need to resort to miracles?
But worse than this disrespect is the pride inherent in these Creationists. In God's greatest Testament, the natural world itself, God has told us how He formed the Earth, and set life loose upon it. Written in the stars is the story of the Cosmos, a tale that someday we might follow back to the beginning of time itself. Written out there for those who have an eye to see. But these Creationists are prideful, and they look down upon the work of God, and instead state that it must have been done this way. They dare to dictate, to God Himself, how the world was formed. Does God need them to approve his methods? Does he need their consent, or their understanding to work with his own power?
These heretics, these blasphemers wish to bring God down to their own primitive level try to recreate God in their own image. If there is a greater sin than that, I cannot name it. God created the World in his own fashion, and God gave us the reason to understand it- as long as we are diligent and wise.