In the New Testament the word "revelation" (apokalupsis) means an uncovering or unveiling.
In scripture, God made known to mankind truth that could not otherwise be discovered by us. We sometimes call the scripture, God's Word, special revelation.
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By contrast, God's creation, nature, that can be seen by all is sometimes called general revelation. Creation testifies to it's Creator, just like a wristwatch points to the watch maker. However, because mankind is fallen our nature, apart from Christ, is to twist the general revelation of God:
"But God shows his anger from heaven against all sinful, wicked people who suppress the truth by their wickedness....Through everything God made, they can clearly see his invisible qualities—his eternal power and divine nature. So they have no excuse for not knowing God.
"...but they wouldn’t worship him as God or even give him thanks. And they began to think up foolish ideas of what God was like. As a result, their minds became dark and confused. Claiming to be wise, they instead became utter fools. And instead of worshiping the glorious, ever-living God, they worshiped idols made to look like mere people and birds and animals and reptiles" (Romans 1:18-23 NLT).
So the witness to God in creation, though powerful, is not enough to show fallen mankind the way of salvation. In fact, it has the effect of leaving everyone worldwide "without excuse."
That's where special revelation comes in. 2 Timothy 3:16 speaks of the scripture as "inspired" or breathed out in a special way by God to insure it communicates His truth accurately.
However, even with the scripture, God's Holy Spirit must act in His convicting ministry on an individaul if they are to recognize it as God's truth and respond. Jesus said of the Spirit:
"And He, when He comes, will convict the world concerning sin and righteousness and judgment" (John 16:8).
And the Spirit guides us as believers "into all truth" by illuminating God's Word to us, helping us understand and live by it (John 16:13).
God's revelation in the Scripture was progressive. That means that over the centuries God revealed more and more truth. As New Testament believers we have revealed truth that Moses, David and the Old Testament prophets did not have.
In turn, Moses had more special revelation than his ancestor Abraham who lived some 400 years earlier.
This is important to keep in mind as we read and study the Bible so we don't read back into a previous passage things only revealed later in Scripture.
By the way, the first hint of the gospel is in Genesis 3:15!