How so? When we say God is omnipresent this indicates the repletive presence of God in all created places and in relation to the limited presence of all creatures.
God's repletive presence means He is incapable of being judged or measured by circumscription or defined by physical limitations or spatial boundaries, but rather identified as filling space or acting upon space while at the same time transcending it. Naturally, God is eternally present everywhere, yet without the finite creation there is no “place.” Thus, theologians refer to God’s eternal presence, or His filling of all things, as His immensity, whereas omnipresence speaks to God’s relation to His creation. God’s immensity and omnipresence are in one respect the same, yet immensity emphasizes God’s transcendence, while omnipresence emphasizes his immanence. This distinction is actually quite important because it clarifies that God was everywhere present at creation of the temporal world. And, now, as the Creator and Sustainer of His creation, God must be present to all space with the fullness of His being in order that it may exist at all.
God, as non-embodied spirit, is everywhere present. God does not extend himself—this would be to divide himself into part, which is impossible—nor does God defuse himself like the sun does its rays. God fills all space. In other words, the limitations of space have no reference to him. He is not absent from any portion of space, nor more present in one portion than another (See Hodge, Theology). God’s omnipresence is not a quantifiable thing but rather it is part of His very nature. God is not present in things as their (the thing's) essence—this would make God pantheistic, but He is present in the fullness of His essence in that He fills and sustains all things.
Contrary to some open theist's odd notions, God, being everywhere present, does not literally come or go to or from specific places. Where such language is employed (e.g., Gen. 11:5; Isa. 64:1–2), it must be recognized for what it is—metaphorical language indicating or invoking a special manifestation of God’s working either in grace or judgment.
Since God is not a physical being who takes up space, it would be wrong to think of God as a sort of gas that fills up the universe. In that sense, He is not everywhere, since God is not a thing, like water or air, that can take up space. Rather, God is everywhere insofar as He is not limited by a spatio-temporal body, knows everything immediately without benefit of sensory organs, and sustains everything that exists. In other words, God’s omnipresence logically follows from his omniscience, incorporeality, omnipotence, metaphysical uniqueness, and role as Creator and Sustainer of the universe. Although neither identical to creation (as in pantheism) nor limited by it…God is immanent, spiritually and personally present at every point of the universe. (See, Francis J. Beckwith, “Mormon Theism, the Traditional Christian Concept of God, and Greek Philosophy: a Critical Analysis,” Journal of the Evangelical Theological Society 44.4 (2001), Questia, Web, 5 Apr. 2012.)
The future, for us, is yet to come. As you agree God knows the future. Why? Because He ordained it, else He would not infallibly know it as a proper object of His knowledge, His fore-knowing. Thanks be to God who knows what is to come such that His glory will be made manifest.
So, again, what reality is being ignored by the Calvinist or the Reformed believer?
AMR