I have been separated from my husband for almost 6 years. Will I still need to seek an Islamic divorce from him if he is not Muslim and denounces Islam?
If you have a concrete evidence that he is non Muslim, there is no marriage bond between you as a Muslim woman and him as non Muslim. When there is no marriage, there will be no question of divorce.
From the time of his becoming non Muslim, the marriage between you and him did end and it became null and void.
After four months and ten days from that date, you will be free to marry a Muslim man if you want.
Wassalam.