Is a high school diploma from Cornerstone Christian Correspondence School a valid for the military?

No. Cornerstone Christian Correspondence School is "accredited" with "Accrediting Commission International" which is not recognized by the state of Georgia or the Department of Education. This means the military and universities with the exception of some community colleges/trade schools) do not recognize the diploma. This is not a matter of universities or the military being "selective" or "picky" or discriminating against religious schools (many of which are legitimately accredited). Universities cannot allow students with a "diploma" from CCCS to receive Title IV funding (such as Stafford Loans and Pell Grants). If a university did allow such a student to attend the institution, it would jeopardize the university's standing with the federal government--no other student would be allowed federal funding which is how most students in the US fund their college education. Separation of church and state is fine, so long as CCCS is upfront about this information which they are not nor is the accrediting body. You would have to take the GED to be allowed into the military (or university), which an education at CCCS (hopefully) will have prepared you for. It is not enough to ask if a school is "accredited." You must take it one step further and ask if they are recognized by the Department of Education. If not, you must carefully consider the consequences of attending.