Eastern Orthodox doctrine holds that "the knowledge of God is vision and immediate understanding of everything, both that which exists and that which is possible, the present, the past, and the future."1
This is consistent with Scripture:
Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.
Psalm 138:16 LXX
My being while it was still unformed Thine eyes did see
This understanding of God's omniscience relates to a certain extent to the Orthodox understanding of time, which generally holds that time as we understand it has no meaning for God. "Foreknowledge of the future is, strictly speaking, a spiritual vision, because for God the future is the present."2
A marginal note in John of Damascus' (676-749) Exact Exposition of the Orthodox Faith (I.XIII) relates God's foreknowledge of the future to the Divine attribute of uncircumscription:
That which is comprehended in place or time or apprehension is circumscribed: while that which is contained by none of these is uncircumscribed. Wherefore the Deity alone is uncircumscribed, being without beginning and without end, and containing all things, and in no wise apprehended3. For He alone is incomprehensible and unbounded, within no one’s knowledge and contemplated by Himself alone. But the angel is circumscribed alike in time (for His being had commencement) and in place (but mental space, as we said above) and in apprehension. For they know somehow the nature of each other and have their bounds perfectly defined by the Creator. Bodies in short are circumscribed both in beginning and end, and bodily place and apprehension.
Orthodox theologian Michael Pomazanski (1888-1988) points out that God's foreknowledge of the future in no way violates our free will. "Just as the freedom of our neighbor is not violated by the fact that we see what he does," he writes, "the foreknowledge of God does not violate the free will of creatures.4
1. M. Pomazanski, Orthodox Dogmatic Theology (3rd ed.), p.69
3. Gregory Nazianzus, Oration 44