Tafsir of Al-Infitar 82:15

"They will enter it" (islayawnaha): This is either an adjective describing the Hellfire, or it is a circumstantial state (hal) of the pronoun referring to the wicked in the predicate, or it is an initiated sentence (ist`naf) based on a question arising from the terror of that situation, as if it were said: "What is their state within it?" Thus, it was said: "They will endure its heat."

Ibn Miqsam read it as yuslawnaha with a shaddah (doubled letter), in the passive voice, meaning they will be caused to enter it. "The Day of Recompense" is the Day of Judgment, which they used to deny, either independently or as part of their denial of Islam.