John 16

As the Savior was just about to leave them, it is obvious that there is much more doctrine and teachings than what is recorded in the four gospels as He said -

"I have yet many things to say unto you, but ye cannot bear them now."   (New Testament | John 16:12)

And then He tells them how He would reveal these things to them -

"Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
14 He shall glorify me: for he shall receive of mine, and shall shew it unto you."  (New Testament | John 16:13 - 14)

Thus there was yet much more knowledge that would be revealed by the Holy Ghost.  In many cases, I believe these to be the mysteries of God and may come to individuals as they are faithful and receive the Holy Ghost for -

"...Verily, verily, I say unto you, Whatsoever ye shall ask the Father in my name, he will give it you.
24 Hitherto have ye asked nothing in my name: ask, and ye shall receive, that your joy may be full."  (New Testament | John 16:23 - 24)

I believe joy comes as we receive the mysteries of God.