As a designation of Christians, means full of faith, trustful, and not simply trustworthy (Acts 10:45; 16:1; 2 Corinthians 6:15; Colossians 1:2; 1 Timothy 4:3, 12; 5:16; 6:2; Titus 1:6; Ephesians 1:1; 1 Corinthians 4:17, etc.).

It is used also of God's word or covenant as true and to be trusted (Psalm 119:86, 138; Isaiah 25:1; 1 Timothy 1:15; Revelation 21:5; 22:6, etc.).

Noah Webster's New International Dictionary of the English Language

1. (a.) Full of faith, or having faith; disposed to believe, especially in the declarations and promises of God.

2. (a.) Firm in adherence to promises, oaths, contracts, treaties, or other engagements.

3. (a.) True and constant in affection or allegiance to a person to whom one is bound by a vow, be ties of love, gratitude, or honor, as to a husband, a prince, a friend; firm in the observance of duty; loyal; of true fidelity; as, a faithful husband or servant.

4. (a.) Worthy of confidence and belief; conformable to truth or fact; exact; accurate; as, a faithful narrative or representation.