However, most people don't take on ID from a theological point of view, so I'll do it here.
ID fails as a solid theological assertion because of its invocation of the God of the Gaps as its controlling deity.
The link above goes to Wikipedia, which states in part that
The God of the gaps refers to a view of God deriving from a theistic position in which anything that can be explained by human knowledge is not in the domain of God, so the role of God is therefore confined to the 'gaps' in scientific explanations of nature.Which describes ID perfectly: evolution is real, they say, but it cannot explain everything -- particularly "irreducible complexity" -- so hey, it must be God.
But the God of the Gaps is an unreliable God because his existence is based entirely on human ignorance. As we push forward the envelope of the Known, the domain of this God inevitably shrinks.
It is unfortunate that over the centuries, as the scientific worldview took root, Christians themselves have continued to support the God of the Gaps as the One True God. For example, Catholics do not declare that a miracle has occurred unless they fail to find another explanation. And they do perform due diligence to find a prosaic explanation, to their credit. But I think they're barking up the wrong tree.
However, this concept of God has had unforeseen consequences. It used to be that you could argue against atheism by pointing to all creation and saying, "Well, how do you explain all this?"
And there was no answer to be given except the ancient Greek idea that the universe has always existed: no creation necessary.
And then came Charles Darwin. Now, don't misunderstand me. I don't think that Darwin was an evil man, nor that he was sent by the devil to deceive us or anything like that. But because the primary God for most of the population was the God of the Gaps, Darwin's theory of evolution was able to provide a plausible alternative to that God.
And many were eager to ditch the God of the Gaps and the whole concept of religion with it, because that meant they were no longer beholden to the theologians, whom they despised. Sometimes rightly.
However, some of those new atheists decided to take the place of the God of the Gaps, embraced the new idea of Progressivism — which proclaimed that human knowledge was sufficient to create a Perfect Society Where Superstition Failed — and the tragic, ongoing consequences of this conceit are documented in Jonah Goldberg's Liberal Fascism. Read the book; learn the lesson.
So, why do Christians continue to "cling" to this unreliable God?
Because many in the scientific community have not confined themselves to science: they have gone ahead and ventured into metaphysics and called it Science. They say that science has obviated the "need for a God," by which they mean that the gap that God formerly occupied has been filled by human knowledge. So don't go pushing your medieval superstitions on We The Enlightened. Separation of Church and State and all that.
The Christians are fighting against this assertion by trying to use science to show that the gap is not filled, which means there is still room for God's existence.
But this is a misguided line of attack for the reasons explained above: it's almost inevitable that the gaps will continue to be filled by human knowledge, and those who rely on the God of the Gaps will be left with nothing to believe in, eventually. Many former believers saw the Gap closing and decided that God was in fact not needed. You can make your own list of the results.
Look, you cannot deduce the existence of God by observing the natural world.
Here's a thought experiment to demonstrate:
Imagine that all trace of human existence vanishes from the Earth: no cities, no houses, no roads, no ruins, no bones. Nothing physical that would indicate that we were ever here. What remains are all the flora and fauna as they currently exist, in the locations where they currently exist.
Then we give it fifty years for the sterile hybrids to die off, and for animals and plants that cannot survive in the wild in their current latitude to die off. Also missing are genetically engineered organisms of any kind.
Then along come some aliens, who set about studying and analyzing the biosphere. One of the things they will discover is that there sure are a lot of different kinds of dogs, and what's more, they're all really wolves.
From this information alone, can they deduce our existence? Because all of the dog varieties are the explicit result of human intervention: we'd have nothing but wolves if we had never existed.
Also consider the many varieties of horse, cow, pig, chicken, sheep, goat, corn, tomato, wheat, potato, rose, clematis, daylily, etc. which are also the result of human intervention.
Can our existence be deduced from these creatures and plants, or can they rightly assume that they are the product of natural evolution. I mean, there's nothing irregular about the DNA. It all obviously happened by natural means.
You are not supposed to be able to deduce God's existence from the natural world. (Why? Two words: plausible deniability.)
Knowledge of God is spiritual knowledge, which can be acquired only through spiritual channels, which means that you cannot really know that God exists except through revelation.
Observe this passage from Matthew 16:
15 He saith unto them, But whom say ye that I am?
16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
17 And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
It may have been possible to deduce that Christ was the Messiah, but any conclusion you arrive at by deduction can also be dismantled by deduction. Not a solid foundation for religious faith.
No, there is no other legitimate basis for faith except divine revelation; all else is unstable, perishable, and changeable. The Bible is a record of revelations that have been given to others, and it is a valuable instrument in helping you develop the desire to have your own revelatory knowledge of the truthfulness of the testimonies that are recorded therein.
But it's not a replacement. Mere interpretation of scripture as a basis for faith is not stable. I was an LDS missionary for 18 months, and we could sit there for hours with Jehovah's Witnesses and "Bible bash," they giving their evidence and we giving ours, but no minds were ever changed. Besides, if the Bible were sufficient, we would not have these dozens and dozens of denominations, all using the same text to come to different conclusions.
I mean, try arguing with a Jewish scholar that the Old Testament testifies of Jesus of Nazareth. You. Will. Get. Nowhere. First, because his level of knowledge dwarfs yours utterly (he knows the original Hebrew). And also because interpretation of a written text alone is not sufficient to reveal all of the Truth.
Nope, it's divine revelation or nothing. Which is why the God of the Gaps is NOT the God of Abraham, Isaac, and Jacob. That God asserts his existence through revelation, not by dwelling just beyond the boundary of human knowledge.
For that reason, I say that if ID is taught in public schools at all, it should be taught under the rubric of Philosophy, and it should be identified correctly as a God of the Gaps theory, and the theological weaknesses of that approach need to be outlined.
And Christians need to abandon the God of the Gaps and embrace the Revealed God, because otherwise they will lose their children to the better arguments of the SecProggs.