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)

I now cite specific examples where the word pistis can only mean faithfulness, fidelity, allegiance:

“When I asked my counselors how this might be accomplished, Haman—who excels among us in sound judgment and is distinguished for his unchanging goodwill and steadfast fidelity (pistei) and has attained the second place of honor in the kingdom—” Greek Esther 3, Addition B: The King’s Letter, 13:3-4, New Revised Standard Version Updated Edition (NRSVUE)

“While these matters were being arranged, a hostile rumor was circulated against the people by some who conspired to do them ill, a pretext being given by a report that they hindered others from the observance of their customs. The Jews, however, continued to maintain goodwill and unswerving loyalty (pistin) toward the dynasty, but because they worshiped God and conducted themselves by his law, they kept their separateness with respect to foods. For this reason they appeared hateful to some,” 3 Maccabees 3:2-4 NRSVUE

“He sent unto them therefore to this effect: King Demetrius unto the people of the Jews sendeth greeting: Whereas ye have kept covenants with us, and continued in our friendship, not joining yourselves with our enemies, we have heard hereof, and are glad. Wherefore now continue ye still to be faithful (pistin) unto us, and we will well recompense you for the things ye do in our behalf,” 1 Maccabees 10:25-27 Brenton’s LXX

I quote two other renderings of the relevant verse:

“Now if you continue to remain loyal to us, we will reward you well.” 1 Maccabees 10:27 Good News Translation (GNT)

“Wherefore now continue still to keep fidelity towards us, and we will reward you with good things, for what you have done in our behalf.” Douay-Rheims 1899 American Edition (DRA)

Further Reading

Abraham: Justified by Faithfulness