Revelation 1:3 (NKJV), “Blessed is he who reads and those who hear the words of this prophecy, and keep those things which are written in it; for the time is near.” Revelation is a book of symbols and prophecies. In order to understand it, and no one will understand it all until it is completely fulfilled, God needs to open a person's heart and mind. Ephesians 1:18, “The eyes of your understanding being enlightened…”
God's method of instruction is first He teaches. Then He waits to see if we use it and apply it to our hearts and lives. As Revelation 1:3 reads, blessed are those who read, hear, and keep God’s instruction. God does not give more understanding until we use what we know. Isaiah 28:9-10 (NKJV), “Whom will he teach knowledge? And whom will he make to understand the message? Those just weaned from milk? Those just drawn from the breasts? For precept must be upon precept, precept upon precept, Line upon line, line upon line, Here a little, there a little.”
There are also specific times when God opens our understanding of prophecy. Daniel prayed to God and said, “I heard, but I did not understand. Then I said, "O my lord, what shall be the outcome of these things?” He (God) said, “Go your way, Daniel, for the words are shut up and sealed until the time of the end. Many shall purify themselves and make themselves white and be refined, but the wicked shall act wickedly. And none of the wicked shall understand, but those who are wise shall understand.” Daniel 12:8-10 (ESV).