“mestama, did not believe in G-d.”
We don’t know that, most secular Jews before the war did believe in G-d, and so are most chilonim nowadays.
There is a debate if a מומר for one or more mitzvahs is considered a Mumat l’Kol haTorah Kulah. But one who worships A”Z and is a מסית is certainly a Mumar.
His halachik status was no doubt:
מורידין ולא מעלין according to all.
Somehow we find in all poskim the status of one who is המיר את דתו in a different category than just a chiloni. We don’t sit shivah for him etc.