Given the linkage between theory and research, science is limited to statements about natural and material reality – about things that are at least in principle observable. Hence, there are entire realms of discourse that science is unable even to address, including such matters as the existence of God.
(p.305, How the West Won, Rodney Stark)
Few things are directly observable. Rather, what we observe are the effects that things have on other things. For example, we don’t observe gravity directly, but rather observe gravity’s effects on various objects.
So, is God observable? If he has effects on other things that in turn we can observe, then yes. Most Christians believe God does have such effects, and in an on-going way. Therefore, the investigation known as science is applicable to at least certain effects of God. Since evidence for God’s effects is evidence for His existence, the matter of the existence of God is – at least to some extent – something that science can address.