faithful, dependable: able to be relied on to keep faith and do what is required
TranslatabilityApproximateCertaintySettled
This is the most common use of the word. It describes someone or something you can count on. A faithful servant carries out what the master entrusted to him. A faithful God keeps every promise he has made. A faithful saying is one you can stake your life on because it will hold true.
The two senses below are the same core idea pointed in two directions. When the word describes a person, it praises proven loyalty: this one came through, kept the trust, did not abandon the post. When it describes words, sayings, or testimony, it certifies reliability: this can be believed because it will not let you down.
The link between them is the idea of trust earned and kept. A faithful person is one whose conduct has shown they will not fail you; a faithful word is a statement whose truth will not fail you. In both, the word marks something that has stood the test and can be leaned on.
Nuances
1adependable in conduct and duty; loyal, proven to keep a trust
believing: one who has put faith in God, a believer
TranslatabilityApproximateCertaintyStrong
In a number of places the same word looks the other way. Instead of describing a person who can be trusted, it describes a person who does the trusting: someone who believes. Here it is the near opposite of ἄπιστος, the word for unbelieving, and the New Testament often uses it almost as a name for a Christian, simply "a believer."
The two ideas sit close together because in the New Testament the person who believes is also the person marked out as loyal to God, and the line between the two is sometimes thin. But the direction is genuinely different. The faithful servant of meaning one is praised for being dependable; the believing woman or the believing children of this meaning are identified by the fact that they have come to faith. A reader usually has to choose which English word fits, and they are not the same word.
Where a verse could be read either way, scholars do weigh it case by case. The clearest believers are the places where the word stands over against unbeliever or faithless, and where it simply labels the members of the church.
applied especially to a saying or testimony that is sure to prove true1Ti 1:15Rev 21:5
Both
both mark something or someone that can be relied on without being let downTit 1:9
Only trustworthy
trustworthy in English leans toward character assessment and rarely describes a statement, where the Greek freely does
How pistos differs from similar Greek words
πίστιςthe noun faith or faithfulness itself, the quality; this word is the adjective for the person or thing that has or shows it
πιστεύωthe verb to believe, the act of trusting; this word is the adjective describing the one who believes or the one who can be believed
ἀληθινόςtrue as opposed to false or counterfeit; this word is dependable as opposed to unreliable, and the two stand side by side in titles like faithful and true
Describes a person who can be trusted to carry out what was entrusted to them, reliable in a duty or stewardship: the good and faithful servant, the one faithful in the smallest matter, the steward who must be found trustworthy.Mat 25:21Luk 16:101Co 4:2
Describes God and Christ as dependable to keep their word and their promises: God is faithful and will not let you be tested beyond what you can bear, he remains faithful even when we are faithless, faithful is the one who calls you.1Co 1:91Co 10:132Ti 2:131Th 5:24
Also carries a sense ancient Greek never gives the other word: believing, a person of faith. It labels someone as one of the believers rather than an unbeliever, a meaning entirely absent from a word for what is real.Act 16:12Co 6:151Ti 4:10
Both
Both certify that someone or something can be relied on and will not deceive, which is why Revelation pairs them as a single title for Christ and a stamp on God's own words: he is called Faithful and True, and the closing visions declare these words faithful and true.Rev 19:11Rev 21:5Rev 3:14Rev 22:6
Marks the real, genuine thing set against a copy, shadow, or counterfeit: the true light behind every lesser one, the true bread from heaven rather than perishable manna, the true vine, the true tabernacle that the earthly one only pictured.Jhn 1:9Jhn 6:32Jhn 15:1Heb 8:2
Names the one real God over against idols and false claimants, and the worship that answers to what is actually so: the living and true God, the only true God, worshipers who worship in truth.1Th 1:9Jhn 17:3Jhn 4:23
pistos in the other biblical languages
אָמַןHebrew“be faithful”the Hebrew root behind firm, reliable, and trusting, which carries both the faithful and the believing sides this Greek word also holds
It also runs the other direction, describing the person who does the trusting rather than the person who is trusted, so it becomes the ordinary word for a believer.Jhn 20:27Act 10:45Act 16:11Ti 4:3
It is applied to God and to Christ, saying that the one who made the promise will keep it no matter what happens to the people who are counting on him.1Co 1:91Co 10:131Th 5:24Heb 10:232Ti 2:13
It can describe a statement rather than a person, marking a piece of teaching as reliable enough to build a life on, and marking words as "faithful and true".1Ti 1:151Ti 3:1Tit 3:8Rev 21:5Rev 22:6
Both
Both describe someone handed a job or a message who carries it through as given, which is why translators reach for "faithful" or "trustworthy" for each.Pro 13:17Mat 24:45Mat 25:21Col 1:71Co 4:2
Both are used of a witness whose report holds up, the person whose account you can act on because it will not bend under pressure.Pro 14:5Rev 1:5Rev 2:13
Both can stand on their own as a name for a group of people, "the faithful", set apart from everyone around them.Psa 12:1Psa 31:23Act 10:451Ti 5:16
Its handful of uses are mostly a complaint about scarcity: dependable people are dying out, cannot be found, or are missing from a whole generation.Deu 32:20Psa 12:1Pro 20:6
It can name the quality itself, dependability as something a generation simply does not have in it, rather than describing a particular dependable person.Deu 32:20
It is used of a whole community rather than only individuals: a town known for keeping faith with Israel, a nation that holds to what it was given.2Sa 20:19Isa 26:2
What scholars say about pistos
Standard lexica split the word along an active and a passive line. The passive use, faithful or trustworthy, is the dominant one and covers persons whose loyalty is proven, God who keeps his promises, and sayings that are reliable. The active use, believing, describes the one who has faith and serves in the New Testament as a near synonym for a Christian, standing opposite the word for unbelieving. A handful of verses can be argued either way, and lexica note the overlap because in the New Testament the believer and the loyal disciple are often the same person.
Sources
BDAG, s.v. πιστόςlexicon
LSJ, s.v. πιστόςlexicon
Bultmann, R., "πιστεύω, πίστις, πιστός," TDNT 6:174-228dictionary