Matthew 13:35 That it might be fulfilled which was spoken by the prophet, saying, I will open my mouth in parables; I will utter things which have been kept secret from the foundation of the world.
Jesus had been speaking many parables about the kingdom of heaven. Matthew records this activity as fulfilling the prophecy of Isaiah. The promise is that the Messiah will speak things which have been kept secret since the beginning of time. In other words, we have a promise that we have heard, and will continue to hear, secrets from God’s Word.
God promises to us that He will reveal things to us that we could not learn or know about from any other source. What things? Gospel words, like the birth, life, death, and resurrection of the Son of God for our forgiveness, life, and eternal salvation.
This “secret” could not be made up by man, discovered by Satan, or heard about in any other way than through a special revelation of God the Spirit through the Word of Gospel. And Jesus loves us enough to tell us the Secrets of the eternal, invisible Kimgdom. And He continues to reveal those secrets to us through the Word read, heard, believed, and applied to our hearts by the Spirit.
This promise does not mean we will know and understand everything about God, His life, and His Kingdom. He will keep some mysteries to Himself. But He wants us to know, understand, and believe what He has revealed. When we are in the Word He will “utter secret things.” It behooves us to read and listen. Deuteronomy 29:29: “The secret things belong to the Lord our God, but the things that are revealed belong to us.”