I was browsing the first epistle of Timotheos, and I noticed that the apostle Paulos sometimes preceded a statement by the Greek phrase πιστὸς ὁ λόγος (cp. 1 Tim. 1:15), which may be translated as "the faithful statement" or "the faithful saying." Why does Paulos do this? What significance does this phrase have? Where and how often does he actually employ this phrase?
Where the phrase is used
Here are several places where the apostle Paul uses this phrase, usually translated by the ESV as "The statement is trustworthy":
(1Tim 1:15 [GNT]) πιστὸς ὁ λόγος καὶ πάσης ἀποδοχῆς ἄξιος, ὅτι Χριστὸς Ἰησοῦς ἦλθεν εἰς τὸν κόσμον ἁμαρτωλοὺς σῶσαι· ὧν πρῶτός εἰμι ἐγώ,
(1Tim 3:1 [GNT]) Πιστὸς ὁ λόγος· εἴ τις ἐπισκοπῆς ὀρέγεται, καλοῦ ἔργου ἐπιθυμεῖ.
(1Tim 4:9 [GNT]) πιστὸς ὁ λόγος καὶ πάσης ἀποδοχῆς ἄξιος,
(2Tim 2:11 [GNT]) πιστὸς ὁ λόγος· εἰ γὰρ συναπεθάνομεν, καὶ συζήσομεν·
(Titus 3:8 [GNT]) Πιστὸς ὁ λόγος, καὶ περὶ τούτων βούλομαί σε διαβεβαιοῦσθαι, ἵνα φροντίζωσιν καλῶν ἔργων προΐστασθαι οἱ πεπιστευκότες θεῷ. ταῦτά ἐστιν καλὰ καὶ ὠφέλιμα τοῖς ἀνθρώποις·
Why and what significance? (a start)
We can observe that the usage is in the "pastoral" epistles.
J.H. Bernard is helpful just with observations, in the Cabridge Greek Commentary Series while commenting on 1 Tim.1:15 [I've used theWord.net software, with these commentaries available here]:
v15. πιστὀς ὁ͂ λόγος. This remarkable formula is peculiar to the Pastorals. Here and in 1Ti 4:9 the words καὶ πάσης ἀποδοχῆς ἄξιος are added; in 1Ti 3:1, 2Ti 2:11, and Tit 3:8 we have the simple form πιστὸς ὁ λόγος. In 1Ti 3:1 it introduces a saying which may well have become proverbial at this stage of the Church’s development, If a man seeketh the office of a bishop, he desireth a
good work. In 2Ti 2:11 the words which follow probably formed part of an early Christian hymn (εἰ γἀρ συναπεθἁνομεν, καὶ συνζήσομεν κ.τ.λ.). In the three remaining cases it refers to some important statement of doctrine tersely and generally expressed (as here and in 1Ti 4:8-9), or with more detail (as in Tit 3:8). πιστός is used in the sense of trustworthy (see below on 1Ti 4:3); and a ‘faithful saying’ in the Pastorals indicates a maxim (whether of doctrine or practice) on which full reliance may be placed. There is nothing in the N.T. quite analogous to the phrase. We have πιστός ὁ θεός (1Co 1:9; 1Co 10:13; 2Co 1:18), πιστός ὁ καλῶν (1Th 5:24), but these do not help us much. A more instructive parallel is afforded by οὖτοι οἱ λόγοι πιστοἰ καὶ ἀληθινοί εἰσιν of Rev 21:5; Rev 22:6. The usual Latin rendering of πιστός in the phrase πιστὸς ὁ λόγος is fidelis; but at this verse r has humanus, a reading also adopted by Augustine in one place. See crit. note on 1Ti 3:1.
Having recently read BDAG lexicon on πιστός, I think what Bernard writes here is a good start for this phrase.