Mark 4:33–34 (CSB)
33 He was speaking the word to them with many parables like these, as they were able to understand.
34 He did not speak to them without a parable. Privately, however, he explained everything to his own disciples.
There are two types of revelation: general and special revelation in relation with how God choses to reveal himself to mankind.
Psalms 19:1-2 (CSB) The heavens declare the glory of God, and the expanse proclaims the work of his hands. 2 Day after day they pour out speech; night after night they communicate knowledge.
Romans 1:20 (CSB) For his invisible attributes, that is, his eternal power and divine nature, have been clearly seen since the creation of the world, being understood through what he has made. As a result, people are without excuse.
Paul will go on to say in Romans 1 that while God's invisible attributes are clearly seen, we suppress our knowledge of God because of born sin and sin choice nature.
Special revelation is God revealing himself in glory through the miraculous. Think of God and Moses, or God and Elijah with the prophets of Baal. The best example is Jesus Christ our Lord.
Now from there, anything in the supernatural realm is revealed by God himself. However our instincts might be honed--that is, whether we believe in the supernatural or not--details of the spiritual will only be revealed if God deems it so.
Are there times when something other than God reveals the supernatural? Yes and it involves angels and demons. Angels will reveal at the command of God. Demons will reveal--demonic oppression or possession--to mankind. But let me argue that demons are under the lordship not of Satan but ultimately under Lord Jesus Christ.
Mark 5:6-8 (CSB) When he saw Jesus from a distance, he ran and knelt down before him. And he cried out with a loud voice, “What do you have to do with me, Jesus, Son of the Most High God? I beg you before God, don’t torment me!” For he had told him, “Come out of the man, you unclean spirit!”
James 2:19 (CSB) You believe that God is one. Good! Even the demons believe—and they shudder.
Whether you believe in the spiritual and the supernatural, you are not going to know it by studying really hard. Ultimately God must reveal it first to you.
This is true with the spiritual. This is true with anything God gives us--the gospel, the Spirit, the Scriptures, and His Son--just to name a few.
1 Corinthians 2:10-16 (CSB) Now God has revealed these things to us by the Spirit, since the Spirit searches everything, even the depths of God. For who knows a person’s thoughts except his spirit within him? In the same way, no one knows the thoughts of God except the Spirit of God. Now we have not received the spirit of the world, but the Spirit who comes from God, so that we may understand what has been freely given to us by God. We also speak these things, not in words taught by human wisdom, but in those taught by the Spirit, explaining spiritual things to spiritual people. But the person without the Spirit does not receive what comes from God’s Spirit, because it is foolishness to him; he is not able to understand it since it is evaluated spiritually. The spiritual person, however, can evaluate everything, and yet he himself cannot be evaluated by anyone. For who has known the Lord’s mind, that he may instruct him? But we have the mind of Christ.