The word “revelation” comes from the Latin revelare, meaning “to unveil.” Greek uses the word apokalupto, (where we get the word “apocalypse”) and Hebrew uses the word galah which also means “to uncover” or “to unveil.” Pulling back a veil to reveal what’s behind it is the key to understanding biblical revelation. The Ark of the Covenant containing the ten words of God (Debarim) or, as they are more commonly known, the Ten Commandments, was kept in the Temple of Jerusalem. The room where it was located was called the Holy of Holies and was separated from the rest of the Temple by a veil. No one could go behind the veil into the Holy of Holies and stand before the Ark except the High Priest, and he was only allowed to do so once a year on the Day of Atonement (Yom Kippur). Revelation was the “unveiling” of the Word of God.
Natural revelation is the process by which ordinary human beings can ascertain certain truths by their own power. Everyone knows fire is hot because everybody has at least felt the heat of a flame. That knowledge is natural revelation. Empirical truth is what can be observed by scientists through observation and experimentation. Philosophical truth is what can be deduced by reason. 2 + 2 = 4 is a self-evident truth. That 2 + 2 = 5 is logical and is philosophically true. The same applies to the reality that if A = B and B = C, then A = C must be true. Likewise, the philosophical principle of non-contradiction means something cannot be its opposite at the same time; for example, you cannot be simultaneously alive and dead, since these terms are mutually exclusive. You’re either dead or alive, but you cannot be both at the same time.
Theological truth, however, is not known by reason or observation, even though it is not necessarily in conflict with philosophy or science. Theological truth is known by divine revelation and is accepted on faith. Faith is believing what cannot be known by human reason alone. Faith is taking the word of someone else without having empirical evidence. The revealed truth that there is one God, but also Three Persons in one God, is a theological and divinely revealed truth. That Jesus Christ is the second Person of that Trinity or that He is one divine Person with two natures, human and divine, is also communicated through divine revelation. It is through faith that we believe in the truth of these divine revelations.
Theologians therefore define revelation as the disclosure by God of those supernatural truths necessary for our salvation. Divine revelation comes from God and can be transmitted through either Sacred Scripture or Sacred Tradition