Concise Bible Dictionary:

This word in both the O. T. and the N. T. is from the same root as “faith.” It is being true to oneself, to one's nature, to any promise given, and to any trust committed. It is in various connections often applied to God Himself (Deut. 7:9; Isa. 49:7; 1 Cor. 1:9; 1 Cor. 10:13; 1 Thess. 5:24; 2 Tim. 2:13; Heb. 10:23; 1 Pet. 4:19; 1 John 1:9). The Lord Jesus also is faithful. He is “a faithful high priest” and a “faithful and true witness” (2 Thess. 3:3; Heb. 2:17; Rev. 1:5; Rev. 3:14; Rev. 19:11). The commandments and testimonies of God are called faithful (Psa. 119:86, 138). The words of the gospel are also faithful: the promises attached thereto will unquestionably be fulfilled (1 Tim. 1:15; 1 Tim. 4:9; 2 Tim. 2:11; Titus 3:8; Rev. 21:5). Christians are exhorted to be faithful as stewards to any trust committed to them, and faithful as witnesses to an absent Lord. "Be thou faithful unto death, and I will give thee a crown of life" (Rev. 2:10).