In Exodus 20 when God came on mount Sinai to talk to His people. They told Moses "let not God speak with us, lest we die" (20:19). 

     After that in Exodus 33 Moses told God to show him His glory. and God reply "you cannot see my face, for no one may see Me and live"(33:20). If a man who talk to God can't see Him, then how could we.

     But thanks to our High Priest (Jesus Christ) who take us above to God. So that we may meet God safely in Prayer.