Six Mysteries God Has Kept A Secret

Are there mysteries that God has kept a secret? Deuteronomy 29:29 says, “The secret things belong to the LORD our God, but the things that are revealed belong to us and to our children forever, that we may do all the words of this law.” This text teaches us that there are realities that the Lord has kept secret and other things that the Lord has revealed to us. What are some of these mysteries? And how are we to approach them? And what are some of the things that God has chosen to reveal to us?