Behold, I will send My messenger, and he shall prepare the way before Me: and the Lord, whom ye seek, shall suddenly come to His temple, even the messenger of the covenant, whom ye delight in: behold, He shall come, saith the LORD of hosts. But who may abide the day of His coming? And who shall stand when He appeareth? For He is like a refiner’s fire, and like fullers’ soap: and He shall sit as a refiner and purifier of silver: and He shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the LORD an offering in righteousness. Then shall the offering of Judah and Jerusalem be pleasant unto the LORD, as in the days of old, and as in former years. And I will come near to you to judgment; and I will be a swift witness against the sorcerers, and against the adulterers, and against false swearers, and against those that oppress the hireling in his wages, the widow, and the fatherless, and that turn aside the stranger from his right, and fear not Me, saith the LORD of hosts. For I am the LORD, I change not; therefore ye sons of Jacob are not consumed.” Malachi 3:1-6 (KJV)
The very assurance that God never changes is assurance that He is trustworthy, and will accomplish all that He has said.