In the Old Testament, God revealed many things to us through the prophets. Today, in both the Old and New Testaments, God is revealing to us through His Holy Spirit. Jesus told us that whatsoever was hidden or covered, shall be made known and revealed. When Jesus asked His apostles who do men say that I am, Peter replied, "You are the Christ, the Son of the living God." Jesus answered and said, "Flesh and blood had not revealed that to you (Peter), but My Father, which is in heaven."
Notice that it was the Father who revealed to Peter that Jesus is the Christ. It is significant to note that the Father revealed it to Peter and not the Holy Spirit. Why? Because this incident happened before Jesus ascended back into Heaven. Thus, the Holy Spirit had not been poured out at that time. Jesus promised to send His Holy Spirit to us, which occurred on the day of Pentecost, 50 days after Christ's ascension. Today it is through the Holy Spirit we receive revelation of the things of God and of the testimony of Jesus - (John 14:26 & 1 Corinthians 2:10-14).
The Book of Daniel also has an interesting prophecy about the end-times. In Chapter 12, an angel told Daniel to shut up the words and seal the book, even to the time of the end when many shall run to and fro and knowledge shall be increased. Most Bible scholars took the meaning of knowledge for the things of the world such as technology, medical, inventions etc. However, we are finding that God is also pouring out knowledge in abundance over His Word. Such is the case with the things hidden from us that were referred to as mysteries. A book by Jonathan Cahn called "The Mystery of the Shemitah" is a prime example of new revelations unfolding for us in these last days just as predicted in Daniel.
These things were always in the Bible, but they were covered or hidden from our knowledge, and therefore they must be made known and revealed to us just as promised. "The Mystery of the Trinity Revealed" is another prime example of a doctrinal truth once hidden from us, but is now revealed to us in these end-times. The best way to find those hidden gems from God's Word is to study and seek for them on your own. However, you can always take a shortcut and get copies of the books that have done all the reseach for you. In any case -- happy seeking and may God bless.