John Calvin on Jude 1:17

17. But, beloved. To a most ancient prophecy he now adds the admonitions of the apostles, the memory of whom was recent. As to the verb μνήσθητε, it makes no great difference, whether you read it as declarative or as an exhortation; for the meaning remains the same, that being fortified by the prediction he quotes, they ought to be terrified.