God cannot lie, it is impossible to his nature to utter a lie or speak something that will not come to pass. Sometimes men promise things and are not able to accomplish their will because circumstances popped up that they had not expected. God is all knowing (omniscient) knowing not only everything happening now, but also knowing all things that will happen in the future in the minutest detail.
- Numbers 23:19 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?
- 1 Samuel 15:29 And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent.
All things God speaks come to pass
- Deuteronomy 18:22 When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
- 1 Kings 8:23 And he said, LORD God of Israel, there is no God like thee, in heaven above, or on earth beneath, who keepest covenant and mercy with thy servants that walk before thee with all their heart: 24 Who hast kept with thy servant David my father that thou promisedst him: thou spakest also with thy mouth, and hast fulfilled it with thine hand, as it is this day.