... having revealed to us the secret of His will, according to His good judgment, which, through Christ, was preplanned for managing1 the fulfillment of times in order to coalesce2 all things in Christ throughout the heavens and the earth.God has a secret. He has a plan that He set forth by the sending of His Son which has now been accomplished. He has hid this plan from the powers of the Earth and humanity. Calvinists often talk about the secret will of God, and I am sure they have this text in mind, but if they do then they have missed something vital about this passage. That is, that God has revealed His secret to us!
This is important in recognizing the soteriological concept of adoption (v. 6). In Christ, we have been made sons of the Father and as such are privy to His will (similar to what Christ says in John 15:15) and thus the biblical concept of mystery is directly tied the biblical concept of revelation. Mysteries in the New Testament do not represent things that are beyond us, but rather things that used to be beyond us that we now understand due to God revealing it to us. This is no different.
In this case, God has revealed to us something about the nature of Christ's rule over the earth. The first century Jewish understanding of the Christ was that He would come in glory, rise to leadership in Judea, and defeat Rome leading to the earthly reign of Judea. However, this was not the case.
The truth is that God's plan in regards to the Messiah was to unite the whole earth together. The Jew and the Gentile are now joined under one head: Christ. This plan was a mystery, a secret, which God has revealed to us.
Try to think of the importance in understanding revelation. It is no little thing that the God of the cosmos has shown us what His plans are. There are no more secrets. There are no more mysteries. We have been brought into the council of God by His good judgment! We can be that close to God!
So many of us reject this privilege, preferring to try and figure God out ourselves. But truly knowing the will of God or the nature of God can only come about if God Himself reveals it to us. We cannot figure out what God is thinking any more than you can right now figure out what I am thinking. It can only happen by revelation.
This is part of our inheritance. Treasure it.
1 'oikonomian' refers to managing a household. Most literally, it would translate as 'house-ruling'. In this case, what is being managed is the timing by which God's plan is fulfilled.
2 'anakephealaiosasthai' means to summarize, or to bring together under one head. In this case, it refers to God bringing everything together so that Christ may rule it as one nation, as opposed to the fractured world that we have now.