Why is the truth hidden from man? I think if man had it he would know?

because people with power are busy trying to conceal it from you, it just doesn't reach you. Sometimes, deep down you already know the truth. You just have to accept it. Denial is what gonna harm you and others

What fun would that be?

Because the truth is not intellectual, and most people look for the truth in the intellectual realm. If you really want to find the truth, lock yourself in your room with no internet for a few weeks and actually face your demons instead of running from them.

Your question is rather vague, and presupposes that we could even agree on what truth is.

The truth is beyond all reckoning. We must live with the shadow of truth. Be content with this cosmic accident called life. Be content with artifice. For that is all we know. Reason, knowledge and morality are just the best we know this moment. No better or worse than we are as we evolve into the future.

The truth is not hidden from this man.

This man does have it and does know.

However it’s a secret, so I can’t tell you what it is.

However if I did, you would do a face-palm and declare with passion “Of course! THAT’S what it is! How silly of me not to have realised all along! That Paul guy sure is smart! And wise!”

Presuming your "truth" ~ = God's Truth, these books claim to provide such truth more plainly (albeit the Bible and other sacred scriptures show truth if one is in the Spirit):

The Path of the Higher Self;
Beams from Meher Baba;
Answers by Mother Meera https://www.mothermeera.com
Autobiography of a Yogi;
Light Is a Living Spirit;
Watch Your Dreams;
The Master of Lucid Dreams;
The Great Divorce;
Testimony of Light;
Runaway Realization.

What do you mean by 'the truth'?

It is not thT case ......

you must show yourself worthy of it by seeking it ..

It's not hidden. We have the Bible. Anyone can read it.