1 John 2:24
 Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you too will abide in the Son and in the Father. (ESV)
When God speaks he means what he says. From the very beginning of Scripture we have God speaking. His authoritative word is what creates the very world that we live in and all of the universes and galaxies that we admire. God's word told the very first couple what was expected of them. His word told them , “You may surely eat of every tree of the garden, but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die” (Genesis 2:16-17).
If the Word of God which Adam and Eve had heard from the beginning had continued to abide in the first couple they would not have fallen into sin and impacted all of humanity. So, what happened? The ancient serpent, Satan himself, questioned what was heard from the beginning! Listen to the words of the serpent in Genesis
 Now the serpent was more crafty than any other beast of the field that the LORD God had made.
He said to the woman, “Did God actually say, ‘You shall not eat of any tree in the garden’?”  And the woman said to the serpent, “We may eat of the fruit of the trees in the garden,  but God said, ‘You shall not eat of the fruit of the tree that is in the midst of the garden, neither shall you touch it, lest you die.’”  But the serpent said to the woman, “You will not surely die.  For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.” (ESV)
The Gospel is un-changing in its truth. There is salvation in Christ and him alone. He is not only the deliverer of the Word, but he IS the Word. He is eternal. He does not change. His word does not change. Be very careful that the Word you accepted is the Word that you follow. If you abide in him you will have eternal life.