The word “secular” refers to civil life, that being; everything under the state or government jurisdiction, that which belongs to the state, especially concerning citizens, their rank, and position in that society/ house/ family. Secular things are affairs of the world; and the definition of which is “living in the world”
Given that secular things “belong to the state”, it will be wise to briefly evidence what the state is.
If something is secular then it is “worldly” and belongs to the state, and the things concerned with the state are: your position in society ie your standing and rank in the state community/ organisation (government), which are the attributes of a “person or thing”.
Definitively then the word secular refers to the community and organisation of state government, which is itself a body politic, which is another word for a country. That which is secular refers to the things which that body politic owns [“belonging to the state”] and has jurisdiction over including its citizenry, who are ranked with a position within that secular society.
See How You Became The Government for further explanation.
Again clearly the word secular is concerned with those persons and things that are “of” and “belong” to the polity/ country/ body politic, which are under its control and governance.
To be clear if you are citizen of a country you belong to said country, and are under its control; being “of the world” by definition.
To summarise:
Secular things are by definition “worldly” things, belonging to the state.
Secular things are earthly not spiritual and exclude the belief in God in maters of ethics and morality.
This fits in with the Strong’s Concordance definition of “world” which means:
Transliteration: kosmos Phonetic Spelling: (kos’-mos) Definition: order, the world
Usage: the world, universe; worldly affairs; the inhabitants of the world; adornment.
(cf. Winer’s Grammar, 26): John 7:7; John 14:27 (); ; 1 Corinthians 1:21; 1 Corinthians 6:2; 1 Corinthians 11:32; 2 Corinthians 7:10; James 1:27; 1 Peter 5:9; 2 Peter 1:4; 2 Peter 2:20; 1 John 3:1, 13; 1 John 4:5; 1 John 5:19; of the aggregate of ungodly and wicked men in O. T. times, Hebrews 11:38; in Noah’s time, ibid. 7; with οὗτος added, Ephesians 2:2 (on which see αἰών, 3); εἶναι ἐκ τοῦ κόσμου and ἐκ τοῦ κόσμου τούτου (see εἰμί, V. 3rd.), John 8:23; John 15:19; John 17:14, 16; 1 John 4:5; λαλεῖν ἐκ τοῦ κόσμου, to speak in accordance with the world’s character and mode of thinking, 1 John 4:5; ὁ ἄρχων τοῦ κόσμου τούτου, i. e. the devil, John 12:31; John 14:30; John 16:11; ὁ ἐν τῷ κόσμῳ he that is operative in the world (also of the devil), 1 John 4:4; τό πνεῦμα τοῦ κόσμου 1 Corinthians 2:12; ἡ σοφία τοῦ κόσμου τούτου, 1 Corinthians 1:20 (here G L T Tr WH omit τούτου); 1 Corinthians 3:19. (τά στοιχεῖα τοῦ κόσμου, Galatians 4:3; Colossians 2:8, 20 (see 5 above, and στοιχεῖον, 3 and 4).)