There are two aspects that I find problematic in the above presentation. Namely, the following two statements:
(1) Evolution is a theory and some people are skeptic about it.
(2) Natural selection is something everybody accepts.

As some people already pointed out, there should be no evolutionary skeptics, because evolution is not a theory, but a fact. One can actually observe evolution, say, in a petri dish, in a population of organisms with higher mutation rates (or some other mechanism that induces variation) over a period of time.
On the other hand, natural selection is a theory. It is the darwinian theory of how evolutionary change happens in a population. The theory of natural selection offers an explanation for how evolution works.
Speciation in the population of finches shows exactly this: that the population changed. It does not show that it changed through natural selection, though this is a good explanation for the observed facts.

The thing to say to an "evolutionary skeptic", in my opinion, is that there are facts on one hand, and personal beliefs in the other hand. The sensible thing to do, when facts contradict your personal belief system, is to revise the latter, and not the other way around.