By REVELATION there was made known to Paul the mystery of Christ, which in other generations was not made known to the sons of men. And it was revealed to God's holy apostles and prophets in the Spirit... what for ages had been hidden in God...
Only God can fully know God, and only God can make Himself known. Revelation is a process whereby God makes Himself known to men.
It is the revealing of something previously hidden, something that is unknown until the moment of enlightenment when it bursts forth from the Spirit of God into the spirit of a chosen person or persons.
It is a self-disclosure of God, a deep knowledge of His person, His purpose, His ways or His plans made known by the Holy Spirit.
Things which eye has not seen and ear has not heard, and which has not entered into the heart of man, all that God has prepared for those who love Him... For to us, God revealed them through the Spirit; for the Spirit searches all things, even the depths of God.
1 Corinthians 2:9, 10
"What spiritual or true Christian does not know the radical difference between the opinions that we form from reading, hearing, and study, and the clear apprehension of truths that are communicated by the direct and inward illuminations of the Holy Spirit?"
"Call to Me and I will answer you and show you great and mighty things -- fenced in and hidden, which you do not know -- do not distinguish and recognize, have knowledge of and understand."