The expression "mystery of iniquity" (more common, "hidden power of lawlessness" - Greek: μυστήριον τῆς ἀνομίας; Lat: mysterium iniquitatis) comes from the Second Letter to the Thessalonians, in a passage where Paul explains that the "Day of the Lord" will not arrive "unless the falling away comes first, and the man of sin is revealed, the son of perdition". (2 Thess 2:1-11 NET)
What makes this passage particularly obscure and hard to understand, is not only the reference to the mysterious "man of lawlessness" and "hidden power of lawlessness" but, most of all, the reference (according to practically all translations) to "what holds him back [to katechon]" (v. 6) and to "who holds him back [ho katechōn]" (v. 7).
I have considered "similar questions" suggested by BE-SE, in particular In 2 Thessalonians 2:7 who is holding back the “secret power of lawlessness”? (Asked 8 years, 6 months ago - Active 2 years ago - Viewed 7k times) and Has Paul's concept in 2 Thessalonians 2:7 been misunderstood? (Asked 10 months ago - Active 3 months ago - Viewed 236 times).
The reason why I ask this question is that traditional translations render the verb κατέχω (katechō) with "to hold back", "to restrain" which are intrinsically transitive verbs (viz. they require an object).
But in both verses (6 and 7), the expressions used, respectively το κατεχον (neuter) and ο κατεχων (masculine), do not have a (stated) object. Isn't it possible that, instead of being elliptical expressions, they are without an object because the verb κατέχω is used intransitively?
So I looked at Thayer's lexicon for Strong's G2722, which only considers κατέχω a transitive verb and, without a shadow of a doubt, that at "2Th ii. 6 sq.", "the power of the Roman empire is meant" and "the one in whom that power is lodged, the Roman emperor". All this with some inconsistency, in particular in the KJV, which gives, respectively, "what withholdeth" [no object] and "who letteth" [no object].
Neither convinced nor satisfied, I looked at something more secular and more authoritative: LSJ A Greek-English Lexicon for κατέχω. As can be easily checked, at the beginning, at § A.I.b. (transitive), sure enough, you find hold back and restrain.
Almost at the end, though, at § B.3. (intransitive) you find the meaning prevail.
If we adopt this intransitive meaning, and translate the verses without assuming the presence of an ellipsis for both occurrences of κατέχω (katechō), we have, tentatively:
6 και νυν το κατεχον οιδατε εις το αποκαλυφθηναι αυτον εν τω εαυτου καιρω 7 το γαρ μυστηριον ηδη ενεργειται της ανομιας μονον ο κατεχων αρτι εως εκ μεσου γενηται
6 And now you know what prevails until its revelation at its proper time. 7 For the secret power of lawlessness is already at work; only, [he is] the one who prevails now till he is taken out of the way.
The only addition I have made is that [he is] (ἐστί). It is quite common in Greek to omit the verb "to be" (εἶναι), when it can be inferred from the context.
I have never found a translation like this, and in particular with this interpretation of the verb κατέχω (katechō).