1 Corinthians 2

Paul told the Corinthians -

"...my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power..."  (New Testament | 1 Corinthians 2:4)

For -

"...we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory..."  (New Testament | 1 Corinthians 2:7)

I find it interesting that Merriam-Webster Dictionary defines mystery as -

"a religious truth that one can know only by revelation..."

This is what Paul is teaching as he writes -

"God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned."  (New Testament | 1 Corinthians 2:10 - 14)

That is why the world can debate religious faith and the nature of God all they want, but it will remain a mystery to them.  It is all about individual faith and worthiness that the mysteries and knowledge of God are received.