Creationists who infer the existence of God using the argument from design reveal their Christian prejudices. Even if we were to assume the intelligent design of, say, humans, application of Occam's Razor in light of current technology would lead to a conclusion of polytheism.
The canonical example is of a watch (the Watchmaker Theory). Observation of a watch would lead to the belief that it had been consciously designed and constructed, since a mechanism of such complex structure and function would not arise by chance in nature. The argument is extended by analogy to biological organisms, humans in particular, or perhaps the earth or the entire universe. Any of these are far more complex than the watch, so, creationists argue, they must have been consciously designed as well.
But a watch is far from the most complex system created by human effort and design. Perhaps a single watch could be the product of a lone watchmaker, but bigger machines, themselves incredibly complex agglomerations of dozens, perhaps hundreds of subsystems each at least as complex as the aforementioned watch, require the efforts of large teams.
Consider a jet aircraft: the complex aerodynamics that allow such a machine to fly take years of study to master. The aircraft's propulsion system is another matter entirely, and it's highly unlikely that the person responsible for designing the turbines is sufficiently expert in chemistry to be able to synthesize the fuel. Then we have electrical systems to worry about, navigation and flight control, landing gear, the list could go on for pages. And the engineers who did the design work likely aren't skilled enough machinists to actually build the aircraft parts to the extreme precision needed.
The Earth, or the human body, is a system far more complex than the most advanced flying machine. If such a system were the product of intelligent design, its creation would be far more plausible as the work of a large group of specialists than as the unique effort of the almighty autocrat of Christian theology. In the rather far-fetched case that we were designed by conscious effort, there was most likely a whole army of gods on the task.