According to Luther, it’s the mastery of the biblical languages. He writes [LW 45:363]
There is a vast difference therefore between a simple preacher of the faith and a person who expounds Scripture [that is, an exegete], or, as St. Paul puts it [I Cor. 12:28–30; 14:26–32], a ‘prophet’. A simple preacher (it is true) has so many clear passages and texts available through translations that he can know and teach Christ, lead a holy life, and preach to others.
But when it comes to interpreting Scripture, and working with it on your own, and disputing with those who cite it incorrectly, he is unequal to the task; that cannot be done without languages. Now there must always be such prophets in the Christian church who can dig into Scripture, expound it, and carry on disputations. A saintly life and right doctrine are not enough.
Hence, languages are absolutely and altogether necessary in the Christian church, as are the prophets or interpreters; although it is not necessary that every Christian or every preacher be such a prophet [exegete], as St. Paul points out in I Corinthians 12[:4–30] and Ephesians 4[:11].
He’s right. And when he’s right, he’s really right. One isn’t an exegete without competence in the biblical languages.