“Απολλυμενοις” is a present participle, which would normally be translated into English as a contemporaneous participle (e.g. “are perishing”). However, the participle can be "broadly antecedent to the time of the main verb, especially if it is articular (and thus adjectival)" (Daniel Wallace, Greek Grammar Beyond the Basics, pp. 625-6). The main verb in the passage is “is hid.” Since the “perishing” condition is the antecedent to the hiding of the gospel, translating “απολλυμενοις” as “are lost” conveys the correct sense.
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