Future contingents cannot be certain to us, because we know them as such. They can be certain only to God whose understanding is in eternity above time. Just as a man going along a road does not see those who come after him; but the man who sees the whole road from a height sees all those who are going along the road at the same time.
Summa Theologiae: Volume 4, Knowledge in God: 1a. 14-18 (ed. Cambridge University Press, 2006) - ISBN: 9780521029124