There are numerous scriptural allusions to the fact that God is invisible. (Colossians 1:15, I Timothy 1:17, Hebrews 11:27, et al.) John wrote that no man has ever seen God. (John 1:18) Jesus Christ told the Jews of his day that they had never seen God's form or heard his voice. (John 5:37) He went on to say that he was the only one who had ever actually seen God. (John 6:46) Indeed, it is implied in several passages of Scripture that a good look at God would destroy a human. (Exodus 33:20-23, et al.)
To be sure, we could point to numerous manifestations of God recorded in Scripture in the form of a burning bush, pillar of cloud, column of fire, or smoking mountaintop or temple. We could also talk about a number of "visions" or mental images of God that were given to different individuals. In similar fashion, we could point to people who heard thunder, trumpets, voices and other loud noises at various times that were associated with the Divine presence.
Nevertheless, Scripture is consistent in its insistence that being in the actual presence of Almighty God is virtually impossible for a human to endure. When Moses drew near to the burning bush, God told him not to get too close and to take off his shoes because of the fact that God's presence had made the surrounding ground holy. (Exodus 3:5) Later, God told Moses to delineate a boundary around Mount Sinai and to execute anyone who dared to cross it. (Exodus 19:12-13) Indeed, while Moses was receiving the Ten Commandments, we are told that the Israelites were afraid to get too close to the perimeter of the mountain because of God's presence. (Exodus 20:18-19) Hence, it is also clear that no one has ever touched God.
So we are left with a God that cannot be seen, heard, touched, tasted or smelled. Unless I have miscounted, I believe that incorporates all five of our human senses - the tools that we employ to perceive the reality around us. Does that mean that God is not a part of our reality? Does that mean that God is not real?
Could our reality be an illusion? Some of the leading physicists and cosmologists alive today have proposed just such a view of reality. Doesn't Scripture state that we exist because God wills us to exist? (Hebrews 1:3) Doesn't God exist simply because "He" is? (Exodus 3:14) So are we real or is God real?