an outsider by lack of training or membership: a private, ordinary person measured against the experts or insiders of some skill, office, or group
The word grows out of a common Greek adjective meaning "one's own, private." Its oldest sense is simply a private person: someone acting in a private capacity rather than holding public office. Every later use narrows that same idea. The word came to name the common soldier next to the general, the layman next to the doctor, the amateur next to the professional, and the one who has not been let in next to the fully initiated member.
So the word never means anything on its own. It always depends on a contrast, and the first question a reader has to ask is what the person is being measured against.
- In Acts the contrast is the trained legal experts of the ruling council, so Peter and John are men with no formal schooling in the Law.
- In 2 Corinthians the contrast is the celebrity public speakers of the day, so Paul concedes only that he does not perform as one of them.
- In 1 Corinthians the contrast is the people who can follow what is being said in unknown tongues, so the person named is the one left standing outside the conversation.
What holds the uses together is position, not ability. The word places someone on the near side of a line: not credentialed, not professional, not inside. It says nothing about how clever or capable that person is, which is why the same word can describe apostles speaking boldly before a court and Paul defending his own knowledge in the very sentence where he uses it of himself.
Nuances
Who the outsider of 1 Corinthians 14:16, 23 and 24 actually is, given that the word stands alone in v. 16 but is paired with ἄπιστος ("unbeliever") in vv. 23-24.
- contestedA believer within the congregation who does not possess the gift of tongues, and so is a layman relative to the πνευματικός; on this reading there is little real difference between ἰδιώτης and ἄπιστος in vv. 23-24.; Heinrich Schlier, ἰδιώτης, TDNT 3:215-217; Hans Conzelmann, 1 Corinthians (Hermeneia, Fortress, 1975)
- contestedAn interested non-member, inquirer or seeker who has not been fully initiated into the community, read against the technical vocabulary of Greco-Roman religious associations.; BDAG, s.v. ἰδιώτης; Gordon D. Fee, The First Epistle to the Corinthians (NICNT, Eerdmans, 1987)
- contestedA positional rather than fixed category: the term marks social standing and place within the Corinthian assembly, so "outsider" language is preferable to any single identity label.; Anthony C. Thiselton, The First Epistle to the Corinthians (NIGTC, Eerdmans, 2000)
- minorityA real social class of sympathetic outsiders with access to Corinthian worship, roughly analogous to the Gentile god-fearers who frequented synagogue worship, rather than a rhetorical construct.; Bradly S. Billings, "The apistoi and idiotes in 1 Corinthians 14:20-25," The Expository Times 127.6 (2016): 277-285