Let me be clear: no one is saved by believing in a young earth, and no one will be sent to hell for believing in millions of years. Our eternal destiny is determined solely on the basis of whether or not we have put our faith in Jesus Christ as our personal Savior and Lord or not. But those Christians who accept the millions of years are being inconsistent in accepting what the New Testament says about the gospel while at the same time rejecting all or some of what the New Testament and Old Testament together say about the age of the earth,
the order of creation events, and the consequences of the Fall of Adam.
Their inconsistency causes many people in our culture and around the world (because evolution and millions of years are taught as fact everywhere) to not trust the Bible and therefore not to believe the gospel. Others may not state it as clearly as some do, but they essentially think the same. If the beginning chapters of the Bible look like history but really are symbolic or mythological, then why should anyone believe the gospels about Jesus, which look like history but are filled with all kinds of miracles that modern science (supposedly) has shown are impossible.