Although I take a specific position in my blog entries, I just want to remind those on this site that my positions are always up for revision or correction.
What does κτίσις mean in Colossians 1:23? The verse most certainly uses κτίσις (the lexical form) as a reference to the known human creation which existed in Paul's time. At the very least, a subset of humanity is being discussed in this passage. While it might seem odd to argue that κτίσις bears this meaning in 1:23, we must note that the long conclusion of Mark's Gospel found in MSS that include aleph, B (et al.) contains a similar use of κτίσις at Mark 16:15. Are we to believe that animals or non-humans are the referents there?
Most importantly Revelation 5:13 proclaims:
καὶ πᾶν κτίσμα ὃ ἐν τῷ οὐρανῷ καὶ ἐπὶ τῆς γῆς καὶ ὑποκάτω τῆς γῆς καὶ ἐπὶ τῆς θαλάσσης ἐστίν, καὶ τὰ ἐν αὐτοῖς πάντα, ἤκουσα λέγοντας Τῷ καθημένῳ ἐπὶ τοῦ θρόνου καὶ τῷ ἀρνίῳ ἡ εὐλογία καὶ ἡ τιμὴ καὶ ἡ δόξα καὶ τὸ κράτος εἰς τοὺς αἰῶνας τῶν αἰώνων.
In the foregoing passage, κτίσις certainly refers to animate rather than inanimate creatures. It does not have reference to the earth or the heavens or to any other inanimate referent that someone might have in mind. Compare Colossians 1:15.
Matthew Poole's Commentary: "Creature with the Hebrews doth eminently signify man, by an antonomasia, or a synecdeche [sic], putting the general for a particular. In the original it is, in all the creature; and so it may be, in all the world, (creature being sometimes used for the system of the world, Romans 8:19-21), in opposition to Judea, i.e. in those other parts of the earth which the Greeks and Romans knew to be then inhabited: under heaven, which is a pleonasm, but of the greatest emphasis, as Acts 4:12."
For more about the rhetorical device, pleonasm, see https://fosterheologicalreflections.blogspot.com/2016/07/redundancy-pleonastic-speech-and-bible.html