By the scriptures, St. Jerome meant those of the Old Testament, or, in this case, Isaiah:
And so, both to you [(Eustochium)], and through you to him [(Pammachius)], I am paying what I owe, complying with the commands of Christ, who says, 'Search the Scriptures' [John 5:39] and 'Seek and you will find' [Matt. 7:7] lest I should hear with the Jews, 'You are in error knowing neither the Scriptures nor the power of God' [Matt. 22:29]. For if, according to the Apostle Paul, Christ is the power of God and the wisdom of God [cf. 1 Cor 1:24], and the one who does not know the Scriptures does not know the power of God and his wisdom, [then] ignorance of the Scriptures is ignorance of Christ.
St. Jerome: Commentary on Isaiah, including St. Jerome's translation of Origen's Homilies 1-9 on Isaiah, trans. Thomas P. Scheck, Ancient Christian writers 68 (New York & Mahwah, NJ: The Newman Press, 2015), 67.According to CCSL 73, the 12th-century Codex Parisinus Bibl. S. Genoueuae 7 reads "ignoratio christi est ignoratio scripturarum."