τῶν τὴν ἀλήθειαν ἐν ἀδικίᾳ κατεχόντων

Says The ungodly are characterized by a present participle: τῶν τὴν ἀλήθειαν ἐν ἀδικίᾳ κατεχόντων, "who … the truth in unrighteousness."

κατέχω (katechō) can mean "hold fast, possess" or "hold back, restrain, suppress." Abbott-Smith lists Rom 1:18 under "possess, hold fast." The BSB, however, renders "suppress" ("who suppress the truth by their wickedness").

Implies "Suppress" fits the argument best. The truth was known: τὸ γνωστὸν τοῦ θεοῦ is φανερόν in them (1:19), and γνόντες τὸν θεόν (1:21). Yet they acted against it. The present tense describes an ongoing stance, and ἐν ἀδικίᾳ names the means or the sphere: by their unrighteousness.

Uncertain Whether "hold (the truth) while living unrighteously" is also possible. It is lexically possible but weaker in context. Either way the knowledge is real, and that is why they are without excuse (1:20).