In the book of Exodus Moses asked to see God’s glory, the conversation is recorded in chapter 33, verses 18-23:
18 And he said, “Please, show me Your glory.”
19 Then He said, “I will make all My goodness pass before you, and I will proclaim the name of the Lord before you. I will be gracious to whom I will be gracious, and I will have compassion on whom I will have compassion.” 20 But He said, “You cannot see My face; for no man shall see Me, and live.” 21 And the Lord said, “Here is a place by Me, and you shall stand on the rock. 22 So it shall be, while My glory passes by, that I will put you in the cleft of the rock, and will cover you with My hand while I pass by. 23 Then I will take away My hand, and you shall see My back; but My face shall not be seen.”
So why can’t we see God’s face, or see him at all? Clearly being denied the sight of God’s face has something to do with his holiness and our sinfulness, but there’s more to it than that, because most of us aren’t even allowed to see God’s “back.” Preservation of free will is the reason God does not allow people to see him. What Atheist, (or anyone else for that matter), would continue in rejecting the God of the Bible, if he visibly manifested himself to them? God performed some special miracles at the institution of covenants, such as those the Jewish nation saw in Exodus, and what the disciples saw in the Gospels. But God generally does not let humanity see miracles, we are expected to come to God through spiritual conviction of sin and the truth, through God’s word (Romans 10:17) and his Holy Spirit (John 15:26).