Paul says that the "Chosen Ones', "Believers" were chosen "before the disruption of the world".

How could Paul possibly know this?

Because he was told of this amazing revelation by the Spirit of God

Think about that..how many millions of years have passed since that occurred, and yet you are one of those chosen.

How would you know that you belong to this select group?

By believing what Paul teaches..it's that simple, and yet  hard to grasp the truth of that statement. 

Surely, anybody who reads  his letters, which contain the Gospel must understand and believe what he says.

At least you would think that, but so many have read his epistles and still don't understand that his gospel is different, and the one that God intends for us to understand. 

But, evidently God has not chosen to give everybody the grace to understand. Without His grace, Paul's teaching remains obscure (not easily seen or understood). 

It must be so, because so few who profess to be Christians do not understand, nor do they believe that Paul's teaching is the sole TRUTH. 

So to we few that are chosen, we should rejoice in the special gift we have received from the Father, even though we have done nothing to warrant it..paulnbob