Normally, people assume that Creationism is unscientific for 4 reasons. First, it is not testable; second, it assumes supernatural powers; third, it is a dogma, based on a holy book; and fourth, it is rejected by scientists. However, from all those reasons you can not really conclude that Creationism is not scientific.

Creationism is just as testable as Evolution theory. From lots of scientific observations, such as DNA sequences and the fossil record, you can very well retrieve sufficient evidence whether all species were instantly created at the same time, or not.

A scientific theory does not automatically exclude supernatural powers. The nonexistence of supernatural powers is just an assumption: we see no hint of their existence, therefore we assume that such powers do not exist. However, if a world were governed by witchcraft, demons and gods as in some science fiction novel, you could do science in such a world as well.

That a theory is based on a holy book is also not an reason for being unscientific, because that book could happen to be correct. If people would worship Newton's Principia Mathematica as a holy book, it would not make its content unscientific.

Being considered as false by all scientists is also no reason because that happened to heliocentrism also.

The main reason why Creationism is considered unscientific is that it lacks two of the four requirements of a scientific theory:

External consistency: the theory may not contradict other established scientific theories; it must lend itself to being fully integrated into the whole of science.

and

Explanatory power: the theory must be able to either fully explain states of affairs hitherto unaccounted-for or serve as an instrument to derive them from more basic states of affairs.

http://unendliches.net/english/naturgesetz.htm