Even though it has been strongly argued that divine foreknowledge doesn't negate free will, allow me to ask the question another way. How could God know our decisions if they are truly free? To know the outcome of something is to imply contingency (and determinism). To put it another way, if a third party can know the nature of an individual then that individual cannot be the author of his nature.
Read another response by Sean Greenberg, Andrew N. Carpenter