This sounds like a non-falsifiable apriori analytic statement I think.
You seem to be saying:
- gaps of knowledge are due to lack of evidence of a material cause.
- identification of a supernatural/spiritual cause requires the same sort of evidence as a natural/material cause.
- Therefore no gap of knowledge can ever be explained by a supernatural cause.
Perhaps this is just a logical tautology then without real world application.
The mistake I believe is in the 2nd line.
The possibility of a supernatural cause requires acceptance of the validity of different types of “evidence” in pursuing true causes from material effects.
If after a 1000 years science still cannot adequately explain, say, levitation by means of a natural cause…then it seems valid enough to me that we can hypothesise the existence of an intangible cause. It is the continued inability of scientists to explain the effect with a tangible cause that constitutes a possibly valid form of “evidence”. Obviously it is not coercive enough to constitute full proof … but it is coercive enough to allow us to start proposing supernatural hypotheses along side natural hypotheses. And this all the more so if we have run out of natural/material hypotheses.
So on the basis of a spiritual/supernatural hypothesis we may test and find a way of predicting levitational likelihood (eg only by very disciplined holy people devoted to a god of popular piety when they pray deeply).
Such testing may well result in more consistent results than any previous material hypothesis. They need not be perfect because if we are dealing with an intangible entity that is personal (ie having arbitrary free will) then sometimes it will act and sometimes, under the same material circumstances, it may choose not to.