The faithfulness to fulfill what one has spoken. Modeled foundationally by God: God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good? (Num 23:19). 2 Corinthians 1:20: For all the promises of God in him are yea, and in him Amen. God's promise-keeping is the bedrock of every other Christian doctrine; if God does not keep His word, nothing remains. The human application is Psalm 15, the psalm of the man who dwells in the LORD's tabernacle: He that sweareth to his own hurt, and changeth not (v. 4). The biblical man's promise binds him even when keeping it costs him personally — precisely the test that distinguishes promise-keeping from convenience-keeping. Christ's teaching in Matthew 5:33-37 raises the bar: rather than negotiating oath-categories, let your yes be yes and your no be no. The Christian who has learned promise-keeping is the Christian whose word stands without oath because his character is reliable.