Well, In Genesis 1, we see that God spoke the universe into being by saying just, "Let it be", and it instantly appeared, fully formed on the same day He said it. And the days were our 24 hour days, not quadzillions of years.
That is not evolution. A person who believes in evolution, does not believe what is written in Genesis 1. That person either has to say that God does not exist, He lied in His account in Genesis 1, or that Genesis 1 was not God-inspired Scripture. The latter brings up a problem because Paul says that all Scripture is God-inspired, so to say that Genesis 1 is not inspired, makes Paul out to be a liar. This puts serious doubts on all the things he wrote, especially the gospel he preached.
Therefore, there are big problems if we say that Genesis 1 is not literally true, that God spoke the universe into being in an instant of time. If God doesn't exist, we have no hope of the future. If He lied, then we can't trust Him in anything else He said in the Scriptures, and if Scripture is not God-inspired then we can't trust Paul either!
Also, if we don't believe one passage of Scripture - if God does exist, He told the truth, and Scripture is God-inspired, then we cannot believe the rest of Scripture either. This means we have no basis for salvation, because we cannot have faith in Christ without believing the whole Bible as God-inspired, including Genesis 1!