τὸ ἔργον τοῦ νόμου γραπτὸν ἐν ταῖς καρδίαις αὐτῶν

Says οἵτινες ("people who") ἐνδείκνυνται, "show" (Abbott-Smith, middle: "to show forth, prove"), τὸ ἔργον τοῦ νόμου γραπτὸν ἐν ταῖς καρδίαις αὐτῶν. ἔργον is singular, against the plural ἔργα νόμου of 3:20, 28. γραπτόν (grapton, "written") is a predicate adjective: they show the work of the law as written on their hearts.

Implies "The work of the law" is what the law requires, its demand taken as a whole, not the written code itself. What these uncircumcised people show is the kind of command named in 2:21–22 (stealing, adultery, idolatry), not circumcision or food law.

Uncertain Whether γραπτὸν ἐν ταῖς καρδίαις alludes to Jer 31:33, which Swete's LXX (Jer 38:33, vol. 3, p. 313) reads καὶ ἐπὶ καρδίας αὐτῶν γράψω αὐτούς. Paul shares "written" and "hearts," but speaks of "the work of the law" and names no writer. Those who read 2:14–15 of Gentile Christians rely on the allusion and its new-covenant setting. Those who read Gentiles generally point to the moral knowledge given with creation (1:19–21, 32) and the accusing conscience of 2:15. This note leans with the second, as in note.language.Rom.2.14.hotan-physei; see also note.language.Rom.2.15.jer-31-33.

Uncertain Whether the singular bears on ἔργα νόμου in 3:20. Dunn (WBC 38A, by reference) takes "works of the law" as the whole law, with the practices that marked Israel off (circumcision, food laws, Sabbath) especially in view. This note leans to the whole law without that emphasis: 2:15 speaks of the law's "work" with no such focus, and 2:21–23 and 3:19–20 name moral commands.