To see why Darwinism and theism are incompatible, consider random mutations and natural selection—the two elements of modern Darwinian theory. Random mutations are, well, random. By definition, random mutations are unguided. "Mutations are simply errors in DNA replication," according to University of Chicago biologist Jerry Coyne. "The chance of a mutation happening is indifferent to whether it would be helpful or harmful." If a mutation is harmful, the organism with the mutation will leave fewer offspring; but if the mutation is beneficial for reproduction, the mutated gene will be passed to many offspring. This is the "natural" selection part. Theistic Darwinists claim that this process creates life's diversity and is also "used" by God.
While theists can have a variety of legitimate views on life's evolution, surely they must maintain that the process involves intelligence. So the question is: Can an intelligent being use random mutations and natural selection to create? No. This is not a theological problem; it is a logical one. The words random and natural are meant to exclude intelligence. If God guides which mutations happen, the mutations are not random; if God chooses which organisms survive so as to guide life's evolution, the selection is intelligent rather than natural.
Theistic Darwinists maintain that God was "intimately involved" in creation, to use Francis Collins's words. But they also think life developed via genuinely random mutations and genuinely natural selection. Yet they never explain what God is doing in this process. Perhaps there is still room for him to start the whole thing off, but this abandons theism for deism.