From the experience of Answers in Genesis, this is one of the primary areas where many Christians (author included for years) have not thought this through to its logical, theological conclusions. But it has powerful consequences related to the gospel.
Here’s the problem in a nutshell: No matter what idea a Christian may use to attempt to squeeze millions of years into the Bible—day age, gap theory, progressive creation, evolutionary creationism, framework hypothesis, and so on—they all have the common problem of putting millions of years of death before Adam’s sin, which is a theological impossibility.
The Bible is explicitly clear, from cover to cover, that death is the consequence of Adam’s sin (Genesis 2:17; Genesis 3; Romans 5:12; 1 Corinthians 15:21–22).1 Death is described as an enemy—the last enemy to be destroyed (1 Corinthians 15:26; Revelation 20:14). But if the idea of millions of years were true, death existed long before man or his sin. This would make God the author of death who looked down on Day 6, after millions of years of death, suffering, and diseases, and called everything “very good” (Genesis 1:31). It is often said, especially to kids, that “God loves you and has a wonderful plan for your life.” But how could people trust that a god who created a world full of death and suffering has wonderful plans for their life? That would not make any sense and many people today, especially kids, recognize this.
This clearly impugns the character of God. And if death were indeed part of God’s original “very good” creation, then why would He describe it as an enemy that would one day be destroyed? On top of that, the future hope of the Christian is that God will restore the world to it’s “pre-Fall” state. But if there were millions of years of death and suffering before man existed, then what will God restore the world to? More death and suffering?
Most important of all, death is presented as the payment for sin (Leviticus 17:11; Romans 6:23; Hebrews 9:22; 1 Peter 2:24). But if you have millions of years and death before sin, then death is not the payment for sin. And if death is not the payment for sin, then Jesus’ death does not pay our sin debt, and we are all still lost in our sins and bound for hell!
Thus the doctrines of atonement and salvation utterly collapse. That’s why the issue of the age of the earth is ultimately a core, foundational issue and not a side issue. And that’s why we at AiG are so passionate about defending the authority of God’s Word, where it’s under assault today, and thus the gospel that’s based in that authority!