Numbers 23:19

"God is not a man, that he should lie, nor a son of man, that he should change his mind. Does he speak and then not act? Does he promise and not fulfil?"

Titus 1:2

"a faith and knowledge resting on the hope of eternal life, which God, who does not lie, promised before the beginning of time,"

Hebrews 10:23

"Let us hold unswervingly to the hope we profess, for he who promised is faithful."

Faithful = pistos (Greek) = worthy of trust; that can be relied upon. God is worhy of our trust and can be relied upon because He keeps His promises!