One must believe in Hellfire, called Jahannam, and that it is already created and that it exists now. Allah ta^ala said in Surat Al Baqarah verse #24:

﴿أعدت للكافرين﴾ سورة البقرة

The verse means: [prepared for the disbelievers.]

It is the strongest and most immense fire Allah created. It exists under the seventh Earth. Hellfire will last everlastingly without an end. Allah ta^ala said in Surat al-Ahzab verse #64-65:

﴿إن الله لعن الكافرين وأعد لهم سعيراَ خالدين فيها أبداً لا يجدون ولياً ولا نصيراً﴾ سورة الأحزاب.

The verse means: [Indeed, Allah has cursed the disbelievers and prepared for them Hellfire. Abiding therein forever, they will not find a protector or a helper.]