Pistos (πιστός) is the adjective form of the pist- word family (cf. pistis, G4102, faith; pisteuō, G4100, to believe). It means faithful, trustworthy, reliable, or believing. It appears about 67 times in the NT and is applied to both God and to human agents who are found worthy of trust.
When applied to God: he is pistos — he keeps his word, his promises never fail, he cannot deny himself (1 Cor. 1:9; 2 Tim. 2:13; 1 John 1:9). When applied to believers: they are those who prove reliable in their stewardship of the gospel, of relationships, and of household duties (Matt. 25:21; 1 Tim. 3:11; Rev. 2:10).
The declaration "faithful is he who calls you" (pistos ho kalōn humas, 1 Thess. 5:24) is the foundation of Christian hope: we do not maintain our standing with God through our faithfulness but through His. This theological anchor allows Paul to pray confidently for the complete sanctification of the Thessalonians — not because they are reliable, but because God is.
At the same time, pistos is the NT's standard for leadership and stewardship. A good steward is found "faithful" (1 Cor. 4:2). The goal of every servant is to hear: "Well done, good and faithful [pistos] servant" (Matt. 25:21). The book of Revelation addresses Jesus himself as "Pistos kai Alēthinos" — "Faithful and True" (Rev. 19:11) — the ultimate model of what it means to be trustworthy to the end.