What is the basis of the argument that Open Theism is heretical?

John Piper has said that [open theism](https://en.wikipedia.org/wiki/Open_theism) (i.e. the notion that God himself changes over time and can choose to be ignorant of His own future if He so chooses) "is an attack, however unwittingly, on the cross and on the work of the Spirit as our only hope of persevering faith and salvation." What is the basis for this assertion?