The late Pastor Bob Trefz refuted this heresy of Open Theism most convincingly:
“The theological problem of freedom versus foreknowledge is resolved immediately when one looks at it from the perspective of God, not man, and from the very name, I AM. God lives in a different dimension than man. God preserves man’s freedom, but because God sees the future like we see the present, He knows in advance the decisions that man, in his freedom, will make. Therefore God knows in minute detail what the future will be and, on that basis, God gave John on Patmos the accurate picture of what will transpire at the end of time.” Cherith Chronicle, March-April 2016, p. 17. Woodland, Washington.
As good as this rebuttal is, we need not rely on the words of a man to resolve this so-called enigma. Scripture and Inspiration can answer this challenge.
“Remember the former things of old: for I am God, and there is none else; I am God, and there is none like Me. Declaring the end from the beginning, and from ancient times things that are not yet done, saying My counsel shall stand, and I will do all My pleasure.” Isaiah 46:9, 10." AdventMessenger