For nothing is hidden, except to be revealed; nor has [anything] been secret, but that it would come to light.
In context, Jesus is talking about parables. He explains the parables to them and goes on to talk about lamps and then this statement comes along. This statement is both generally true and specifically true.
This is specifically true, because God reveals things to those who seek Him. The Bible contains promises that the Holy Spirit will open up the contents of the scriptures to believers who ask. There is nothing hidden in the scripture that God will not eventually make plain as day. All prophecies will be fulfilled. All mysteries will have an unveiling. Eventually, everything will be simple and plain. We will see Him as He is.
This principle is also generally true. We do not hide things unless we want to reveal them to someone. And secrets are truly difficult things to keep. These are even more true when the thing we are hiding or the secret we are keeping is good. Trying to hold back the announcement when my wife and I found out we were expecting a child better than a year ago was difficult, but the information was withheld because we wanted it to come to light at a particular moment and to reveal it in a certain way.
God keeps certain things secret for reasons that make sense to God — and might to us, too, but we will not know until God reveals both the secret things and His reasons for keeping them secret. God keeps certain things hidden. He does not do this because He is secretive or has anything to hide. God has reasons for keeping certain things hidden, just as we do. But nothing is hidden except because there is a right moment for it to come to light.
Father God, please let me be open to receive whatever You would reveal today. Let me not begrudge You the secret things, but rejoice in those things that You determine I am ready to hear and understand. Thank You that there will come a day when I will be able to understand it all. Today is not that day, but that day is coming and I look forward to it with eagerness.