Yes, of course, God is omnipresent. But this is spoken of not only in biblical revelation...
Yes, of course, God is omnipresent. But this is spoken of not only in biblical revelation; pantheistic religions are very ancient. But the God to whom the psalmist turns has astonishing differences from the god of pantheistic religions: He is Spirit, but He has a face turned toward me and a hand with which He leads and holds me. God has personal relationships with me, relationships that are not broken on His side even when I avoid them. And one more thing: to descend into the underworld means to die, to go away from God forever, into nonbeing. But God comes even there, in order to bring even nonbeing into being.