We can know that God is faithful because...
“What comes into our minds when we think about God is the most important thing about us.” (A W Tozer - The Knowledge of the Holy)
Psalm 119:105 Your word is a lamp to my feet and a light to my path.
Exodus 34:6 The LORD passed before him and proclaimed, “The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness.
Psalm 36:5 Your steadfast love, O LORD, extends to the heavens, your faithfulness to the clouds.
Proverbs 3:5–6 Trust in the LORD with all your heart, and do not lean on your own understanding. In all your ways acknowledge him, and he will make straight your paths.
Proverbs 16:9 (HCSB) A man’s heart plans his way, but the Lord determines his steps.
Romans 8:28 And we know that for those who love God all things work together for good, for those who are called according to his purpose.