To answer this, I'm taking info directly from The New Answers Book 1 from Answers in Genesis, edited by Ken Ham, because it's so clearly presented there's no point in my re-doing it. All quotation of the book will be italicized. So, beginning on page 26...
...some of the reasons we think that Christians cannot accept millions of years without doing great damage to the church and her witness in the world.
1. The Bible clearly teaches that God created in six literal, 24-hour days a few thousand years ago. The Hebrew word for day in Genesis 1 is yom. In the vast majority of its uses in the Old Testament it means a literal day; and where it doesn't, the context makes this clear.
2. The context of Genesis 1 clearly shows that the days of creation were literal days. First, yom is defined the first time it is used in the Bible (Genesis 1:4-5) in its two literal senses: the light portion of the light/dark cycle and the whole light/dark cycle. Second, yom is used with "evening" and "morning." Everywhere these two words are used in the Old Testament, either together or separately and with or without yom in the context, they always mean a literal evening or morning of a literal day. Third, yom is modified with a number: one day, second day, third day, etc., which everywhere else in the Old Testament indicates literal days. Fourth, yom is defined literally in Genesis 1:14 in relation to heavenly bodies.
3. The genealogies of Genesis 5 and 11 make it clear that the creation days happened only about 6,000 years ago. It is transparent from the genealogies of Genesis 5 and 11 (which give very detailed chronological information, unlike the clearly abbreviated genealogy in Matthew 1) and other chronological information in the Bible that the Creation Week took place only about 6,000 years ago.
4. Exodus 20:9-11 blocks all attempts to fit millions of years into Genesis 1. "Six days you shall labor and do all your work, but the seventh day is a sabbath of the Lord your God; in it you shall not do any work, you or your son or your daughter, your male or your female servant or your cattle or your sojourner who stays with you. For in six days the Lord made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therfore the Lord blessed the sabbath day and made it holy" (Exodus 20:9-11)
This passage gives the reason for God's command to Israel to work six days and then take a sabbath rest. Yom is used in both parts of the commandment. If God meant that the Jews were to work six days because He created over six long periods of time, He could have said that using one of three indefinite Hebrew time words. He chose the only word that means a literal day, and the Jews understood it literally (until the idea of millions of years developed in the early nineteenth century). For this reason, the day-age view or framework hypothesis must be rejected. The gap theory or any other attempt to put millions of years before the six days are also false because God says that in six days He made the heave and the earth and the sea and all that is in them. So He made everything in those six literal days and nothing before the first day.5. Noah's Flood washes away millions of years. The evidence in Genesis 6-9 for a global catastrophic flood is overwhelming. For example, the Flood was intended to destroy not only all sinful people but also all land animals and birds and the surface of the earth, which only a global flood could accomplish. The Ark's purpose was to save two of every kind of land animal and bird (and seven of some) to repopulate the earth after the Flood. The Ark was totally unnecessary if the Flood was only local. People, animals, and birds could have migrated out of the flood zone before it occurred, or the zone could have been populated from creatures outside the area after the Flood. The catastrophic nature of the Flood is seen in the nonstop rain for at least 40 days, which would have produced massive erosion, mud slides, hurricanes, etc. The Hebrew words translated "the fountains of the great deep burst open" (Genesis 7:11) clearly point to tectonic rupturing of the earth's surface in many places for 150 days, resulting in volcanoes, earthquakes, and tsunamis. Noah's Flood would produce exactly the kind of complex geological record we see worldwide today: thousands of feet of sediments clearly deposited by water and later hardened into rock and containing billions of fossils. If the year-long Flood is responsible for most of the rock layers and fossils, then those rocks and fossils cannot represent the history of the earth over millions of years, as evolutionists claim.
"Okay, so you've addressed literal translation of Genesis, but you still haven't answered why this is sooooo important?" Next week I'll complete posting the AiG answer to this question which addresses Jesus' position, the Bible's authority, and faulty science. And if you want more information on this, read The New Answers Book 1. They go in to much more detail in answering these questions and others than I have time for here.
