singleness of heart: an undivided, sincere inner life with no hidden motive, which in giving overflows as open-handed generosity
The picture inside this word is something laid open rather than folded over on itself. Its root adjective (ἁπλοῦς) is the opposite of the word for "double," so a person who has it is not two-faced or self-divided: what they show is what they are. The Greek Old Testament reached for this word to translate Hebrew terms for a whole, upright heart, and that is the meaning it carries everywhere in the New Testament: sincerity, integrity of motive, single-minded devotion.
The one stretch where readers stumble is Paul's letters about the collection for the poor, where most modern Bibles translate it "generosity" or "liberality." The two senses below are not two different words. They are one disposition seen from two angles. The undivided heart is, by its very nature, the open hand: where the inner life is singular and unmixed, the giving is whole-hearted and ungrudging. So in the giving passages Paul names the inward quality in order to praise the outward act it produces.
This is why translators are caught between options. "Sincerity" keeps what the word says but sounds odd in a sentence about money; "generosity" reads naturally in that sentence but loses the note of an unmixed heart. The word itself does not force the choice, because for Paul the single heart and the open hand are the same heart and the same hand.
Nuances
Disputed
Whether ἁπλότης has a second, autonomous lexical sense "generosity / liberality" in Paul's collection passages (2 Corinthians 8:2; 9:11, 13; Romans 12:8), or whether those verses are the same single-hearted-sincerity meaning applied to a giving context.
- majorityOne meaning: "generosity" is a contextual extension of single-hearted sincerity, not a separate lexical sense. The undivided heart is named to denote the open-handed action it produces.; Joseph Amstutz, ΑΠΛΟΤΗΣ (1968); BDAG, 3rd ed. (2000), s.v. ἁπλότης; Ceslas Spicq, Theological Lexicon of the New Testament; Otto Bauernfeind, TDNT vol. 1; Mark A. Seifrid, The Second Letter to the Corinthians (Pillar, 2014); C. E. B. Cranfield, The Epistle to the Romans (ICC)
- minority"Generosity" is a genuinely distinct Pauline sense: in the collection contexts the word denotes the concrete virtue of generous, open-handed giving as a real second meaning.; Louw-Nida, Greek-English Lexicon, 57.106; Robert Jewett, Romans (Hermeneia, 2007), on Romans 12:8