“Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself: That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:”
What is the “mystery of His will“?
There are many mysteries of the Kingdom, some of them are about The First Earth Age, where Satan rebelled against our Father. While many people took the side of Satan during that time, some people actually stood with our Father and it is those individuals who are “predestined” as Romans 8 declares.
“Predestined” for what?
To make a stand with our Father, to make an example for others during our time on earth and for those who live during the last days, to stand against Satan once again.
“The dispensation of the fulness of times.” I believe this is a reference to the end of this age of flesh. When Christ returns all things will be gathered together under Christ. Those who have passed on before us “in heaven” and those of us who are still “on earth“.
At the Seventh Trumpet, those of us on earth will instantly be changed back into our true bodies which are our spiritual bodies that never get sick, old or die. It is at that moment in time that we are all gathered back to Christ and what a day it will be (1 Thessalonians 4:13-17, 1 Corinthians 15:51-57)!
Bible Q&A: What Is The Mystery Of God?