Will one who believed in Allah and His Messenger in their heart, but did not utter the Two Testimonies and committed major sins until death, enter the Fire and abide therein eternally?

The faith of one who does not pronounce the two testimonies (Shahadatayn) while being able to do so is not valid. Their destiny in the Afterlife is eternal Hellfire. Their mere knowledge of the truth of the content of the two testimonies does not benefit them, for inner conviction alone does not benefit its possessor unless it is followed by outward submission to Islam. One is not a believer except by the combination of inner conviction and outward affirmation. So, whoever abandons submission is arrogant, like Iblis, even if they are a firm believer. Thus, disbelief is more general than mere denial.