...according to the riches of his grace, which he lavished upon us, in all wisdom and insight making known to us the mystery of his will, according to his purpose, which he set forth in Christ as a plan for the fullness of time, to unite all things in him, things in heaven and things on earth.
We see in these verses Paul’s continued sense of wonder at God’s eternal purpose, a wonder that all the Lord’s people should develop.
It is not clear to whom ‘all wisdom and insight’ refers. It is possible that it refers to how God’s grace works, that is, it is wise grace, and if this is the meaning it refers back to the previous clause. Or it could refer to the following clause, how believers ‘know’ the mystery of his will, that is, they are enlightened about it. I suspect the latter is meant, because Paul prays in verse 17 that further wisdom would be given to them. So we are equipped by God to understand his purpose. That is something to be grateful for!
Paul describes God’s eternal purpose as a ‘mystery’. This term ‘mystery’ does not mean something that is mysterious or secret; rather it refers to something in the plan of God that was previously hidden but which is now revealed in the preaching of the gospel.
There have been two interpretations of ‘a plan for the fullness of time’. One is that the New Testament period is meant, and that the unity referred to is the unity of angels (things in heaven) and humans (things in earth) in the church. It is difficult to conclude that all Paul refers to by ‘all things’ are the angels and the church.
The other interpretation, which I think is the correct one, is that the state of perfection at Christ’s second coming is intended here by Paul and describes the renewal of the cosmos after the day of judgement. The day is coming when universal harmony will be restored by Jesus Christ when he brings the new heavens and new earth into existence, in which he will dwell with his people. That is something to look forward to!