I have no desire to be contentious, however your claim is unfair to Plantinga, and in the interest of fairness, I should challenge it further.
...so, you have an individual, Alvin Plantinga, saying that people can know a god exists by belief alone.
Remember I said that it's easy to caricature, misrepresent and then mock what Plantinga actually says? Well....
I don't entirely blame you, though. It's an easy mistake to make one has actually never read what he wrote, and particularly if one does not understand the philosophical term "properly basic" or Plantinga's explanation for how one ought to regard beliefs as "properly basic." If one already thought he knew what "properly basic" entailed, or if one substituted common language for Plantinga's more complicated stipulated concept, then one would be quite likely to go badly wrong in critique. The solution is simple: read what he actually wrote, not the dilatory summary of some lesser interpreter on (gasp) Wikipedia, and make up one's own mind.
Proof is empirical. Not based upon belief. Belief is not proof.
If you read a bit in epistemology, and particularly in the philosophy of science, you will find that this claim is epistemologically naive, from two perspectives. Firstly, even science itself is inductive, not deductive, as every philosopher of science knows; and "empirical" means, "derived from sensory inputs produced by experience or experiment." Both experience and experiment are only capable of producing
probabilistic results, not "proof" in any pure sense. "Proofs" happen in mathematics and formal logic only, not in science. "Proof" is not empirical. "Evidence" is empirical -- and probabilistic.
Secondly, your claim is naive about belief. Belief is the action of estimating the probability of the veracity of a claim, and therefore science does it all the time: it looks at the probabilities that its empirical findings will continue, and ventures a conclusion (tentatively) based on best evidence. It "believes" its results.
The idea that Atheists possess some kind of "proof" and all that Theists have instead is some kind of "belief-minus-anything" is simply naive on both counts.