hidden, concealed, out of sight; by extension, done in secret or kept from view.
This is an ordinary Greek word for anything kept out of sight, and it stayed that one word from the oldest Greek poetry through the New Testament. It can describe a thing physically covered, an action done where no one can watch, or information no one else knows. The same word lies behind our "crypt," a covered vault. So at its core it simply means hidden, and the various shades below are that one idea bent to different situations, not different words.
The New Testament gives this plain word a striking theological job. Again and again it sets "hidden" against "made plain," and turns the pair into a promise: whatever is hidden now will one day be brought to light. The God who sees in secret is the one who does the revealing. So a word that just means concealed becomes the hinge of a claim about how God deals with people: nothing stays covered before him, and at the end he uncovers it all.
A few times the word edges toward the idea of the inner self, the part of a person that other people cannot see. It never stops meaning hidden when it does this. The interior sense comes from what it is paired with, most clearly "the hidden person of the heart," rather than from the word taking on a brand new meaning. The hidden self is simply the self that is out of sight to everyone but God.
Nuances
Whether en to krupto in this verse means the inner self of the true Jew, or names the Jew whose Jewishness is hidden from public view and will be vindicated by God in the end.
- contestedIt functions as inwardly: the heart is the location of true covenant identity, in line with the biblical theme of a circumcised heart.; C. E. B. Cranfield, Romans (ICC; Edinburgh: T&T Clark, 1975), 1:175-176; D. J. Moo, The Epistle to the Romans (NICNT; Grand Rapids: Eerdmans, 1996), 175-176; T. R. Schreiner, Romans, 2nd ed. (BECNT; Grand Rapids: Baker, 2018), 142-143
- contestedIt means the hidden Jew, one whose identity is not publicly visible and who awaits the praise that comes from God rather than people; reading it as inwardly imports a later body-and-soul dualism.; R. Jewett, Romans (Hermeneia; Minneapolis: Fortress, 2007), 235-237; M. Thiessen, Paul and the Gentile Problem (Oxford: Oxford University Press, 2016); N. T. Wright, Paul and the Faithfulness of God (Minneapolis: Fortress, 2013), 1117-1119