“Therefore I tell you, do not be anxious about your life, what you will eat or what you will drink, nor about your body, what you will put on. Is not life more than food, and the body more than clothing?” (Matthew 6:25 ESV)
The promises of man are not sure to be fulfilled.
With the best wishes and intentions, he cannot always keep his word. Disease and death may step in like an armed man, and take away from this world him that promises. War, or pestilence, or famine, or failure of crops, or hurricanes, may strip him of his property, and make it impossible for him to fulfil his engagements.
The promises of God, on the contrary, are certain to be kept. He is Almighty: nothing can prevent His doing what He has said. He never changes: He is always “of one mind”: and with Him there is “no variableness or shadow of turning.” (Job xxiii. 13; James 1. 17.)
He will always keep His word.
J.C. Ryle, Holiness: Its Nature, Hindrances, Difficulties, and Roots, p. 246, Kindle Edition.