All of God is everywhere at all times.
In the great dedicatory prayer of the first temple of God, King Solomon said, “But will God indeed dwell on the earth? Behold, heaven and the highest heaven cannot contain you; how much less this house that I have built!” (1 Kings 8:27). The temple was God’s dwelling place among His people. But Solomon knew that God could never be contained in any building. The wise king knew that God is omnipresent. That is to say, He is all-present.
Since God is Spirit (Jn. 4:24), He is not confined to space as we are. He is not “made up” of matter. In fact, He is not “made” at all. God existed before all things were created and all things that were created exist in Him as He holds them together (Col. 1:17; Gen. 1:1). This means that God is outside of creation and creation is inside of Him in some mysterious way. In other words, God must have existed before time, space, and matter since all three were created by Him and none would exist without Him (Gen. 1:1). With this in mind, it is easy to see why God would necessarily be omnipresent.
Still, being present in all places is not the full meaning of the doctrine. The Bible teaches that God is present in three ways. First, God is present in all places (Jer. 23:23-24). Since all things exist in Him, then there is no thing that exists where He is not. God is everywhere. Second, God is present at all times (Revelation 4:8). Since He is outside of creation (including time) then He necessarily can be in the past just as much as the present and in the future just as much as the past. God is not just everywhere, He is in every time.
The third way to think about God’s omnipresence might be the most important. Everywhere that God is, He is with all of His nature. With our finite mind, it is hard for us to think about being in multiple places without thinking in terms of parts. For instance, a family might enjoy a slice of the same pizza and say that each of them has pizza. But none of them can say that any of them has the entire pizza. God, being spirit, is not separating Himself to be here and there. The fullness of God is everywhere that He is (Isaiah 6:3).
There is no place where God is not. He is everywhere that creation is and He is further. And, where ever God is, He is fully there. King David wrote this about God’s omnipresence:
Where shall I go from your Spirit?
Or where shall I flee from your presence?
If I ascend to heaven, you are there!
If I make my bed in Sheol, you are there!
If I take the wings of the morning
and dwell in the uttermost parts of the sea,
even there your hand shall lead me,
and your right hand shall hold me.
1 Kings 8:27