Pistis = Faithfulness

Here is how A Greek-English Lexicon of the New Testament and Other Early Christian Literature (commonly referred to as BDAG) defines the Greek word employed within the writings of the New Testament:

πίστις, εως, ἡ ⟦pístis⟧ (Hes., Hdt.+; ranging in meaning from subjective confidence to objective basis for confidence). 1 that which evokes trust and faith a the state of being someone in whom confidence can be placed, faithfulness, reliability, fidelity, commitment (X., An. 1, 6, 3; 3, 3, 4; Aristot., Eth. Eud, 7, 2, 1237b, 12; Polyb. 7, 12, 9; 38, 1, 8 al.; Herodian 2, 14, 4 al.; SIG 675, 22; OGI 557, 16; PTebt 27, 6; 51 [II b.c.]; POxy 494, 9; 705, 32; other pap M-M. s.v.; Ps 32:4; Pr 12:22; Jos., Ant. 2, 61; TestAsh 7:7) w. κρίσις and ἔλεος Mt 23:23. (Opp. ἀπιστία as Hes., Op. 370) τὴν πίστιν τοῦ θεοῦ καταργεῖν nullify the faithfulness/commitment of God (cp. Ps 32:4; Hos 2:22) Ro 3:3. πᾶσαν π. ἐνδείκνυσθαι ἀγαθήν show all good faith( fulness) Tit 2:10 (cp. BGU 314, 19 μετὰ πίστεως ἀγαθῆς). W. other virtues Gal 5:22 (on πίστις, πραΰτης cp. Sir 45:4; 1:27). W. ὑπομονή 2 Th 1:4. τὴν πίστιν τετήρηκα I have remained faithful or loyal (πίστιν τηρεῖν as Polyb. 6, 56, 13; 10, 37, 5; Jos., Bell. 2, 121; 6, 345; OGI 339, 46f; IBM III, 587b, 5f [Dssm., LO 262=LAE 309, esp. note 3]) 2 Ti 4:7, though this would be classified by some under 3 below. S. also 1c below. b a solemn promise to be faithful and loyal, assurance, oath, troth (X., Cyr. 7, 1, 44; 8, 8, 3, Hell. 1, 3, 12; Diod S 14, 9, 7; Appian, Bell. Civ. 4, 86 §362 μεγάλας πίστεις ἔδωκεν=solemn assurances; 3 Macc 3:10; Jos., Ant. 12, 382) τὴν πρώτην πίστιν ἠθέτησαν 1 Ti 5:12 (s. also ἀθετέω 1 and cp. CIA app. [Wünsch, Praef. p. xv] of a woman who πρώτη ἠθέτησεν τὴν πίστιν to her husband). Cp. Rv 2:3. (BDAG, edited and revised by Frederick W. Danker [University of Chicago Press, Fourth Edition, Chicago, IL 2021], p. 726; emphasis mine)

Further Reading

Abraham: Justified by Faithfulness