Karl Popper
"Deism"
represents a good methodological example of what Popper ("The
Logic of Scientific Discovery", 1934 and "Conjectures and Refutations",
1963), called an "untestable hypothesis". That is, a proposition which
has the appearance of an hypothesis but which cannot be tested
against evidence and, in consequence, can neither be "proven"
or, more-correctly according to Popper, potentially "falsified".
In this respect, it is evident that whether or not you choose to believe in God is an act of faith; that is, the existence of God can neither be proven nor falsified - you either believe or you do not believe.
Popper argued that for any theory to be potentially more than a simple act of faith, it had to be capable of - or open to - the possibility of falsification (that is, being shown, demonstrably, to be false). In other words, for Popper, "science" was viewed as a methodological process involving a set of clear rules that, through their systematic application, enabled us to produce reliable, valid and verifiable knowledge.