Paul continues to address the Corinthians' pride regarding who they have received their understanding of who Jesus is and what He taught by giving them a sober reminder that they don't know as much as they think they do. Their immaturity was demonstrated by believing they had all the knowledge they needed. He calls them to return to reality by appreciating how much more they need to learn and appreciate about the simplest of truths that it will take an eternity to fully appreciate.
Maturity is a healthy understanding of how much more you need to know.
A Mystery from God isn't something hidden and unknown, but something continuing to be revealed.
The Holy Spirit is the only reason we are aware of our need for Jesus. We also need Him to grow closer to Him.