Classical theism describes God as complete, unchanging and unaffected by the world God made, perfect in a way that leaves no room for growth, since anything capable of change was assumed capable of change for the worse. Alfred North Whitehead, a mathematician who turned to philosophy in his sixties, rejected the picture of reality on which that description rests. Reality, Whitehead argued in Process and Reality, is not made of fixed substances that occasionally undergo change but of momentary events of experience, each one arising out of the events before it and passing its own achieved value on to the events that follow, so that becoming, not static being, is what is actually fundamental.
Applied to God, that picture produces something close to a reversal of the classical account. Whitehead's God has two aspects: a primordial aspect that holds every possibility open as a lure toward value, and a consequent aspect that takes the actual, achieved experience of the world back into itself and is genuinely changed by it, feeling what the world has actually become rather than surveying it from an untouched distance. Charles Hartshorne, Whitehead's student, spent his career drawing out what this meant for theology, arguing that a God capable of being moved by the world's joy and its suffering is a more perfect God, not a less perfect one, than an unmoved absolute could ever be. That argument became process theology, the school's most consequential export into twentieth and twenty first century Christian and Jewish thought.