The Gospel is simple, and praise God for it. Yet, many preach false gospels, and many who even affirm the true Gospel at face value don't affirm it on God's terms. An altered gospel is a false gospel. It's God's Gospel, therefore man has no right to change it or reject it.
Scripture References: 1 Corinthians 15:1-11
Several stand alone sermons from The Response Church
1 Corinthians 15:1-11