One more bit from Letham:
Indeed, the Christian faith can be summed up as, inter alia, a series of unions. There is the union of the three persons in the Trinity, the union of the Son of God with our human nature, the union of Christ with his church, the union established by the Holy Spirit with us as he indwells us. Each of these unions preserves the integrity of the constituent elements or members, being at once a real union and simultaneously not absorbing the one into the other.
—Union with Christ: In Scripture, History, and Theology, pp. 37-38
You could spend the rest of your Christian life simply studying the various unions mentioned by Letham and still not come to the end of the grace and glory of the Gospel. The Triunity of God alone ought to, and will, delight us for all eternity, before we even get to the unions whereby God includes us in that life, becomes our brother, makes us part of the church, or sets us free from sin. Simply put: the Gospel is a union thing.
Soli Deo Gloria
Reformedish 
The Gospel is indeed a Union thing. But that leaves me with a niggling question: Is sin, then, a Union-buster?
It’s kind of a serious question. What does Letham say about the presence of sin in a believer’s life? Does it diminish union? hinder it? threaten to break it? or does it only rob us of our enjoyment of it?
I haven’t gotten there yet but I would suspect that he would say, “No, it is not a union-buster.” I would invoke John Owen’s distinction between union and communion. Union is permanent, but communion is our enjoyment of that union. That indeed can be grieved and wounded by sin.
Thanks for sharing this passage from Letham’s volume on Union with Christ. Another great book of the same title is J. Todd Billing’s Union with Christ: http://www.amazon.com/Union-Christ-Reframing-Theology-Ministry/dp/0801039347
I loved that book. It was one of my top 5 of 2012.
I would have to say it was for me as well. I especially appreciated his chapter “Union with Christ, the Law of Love, and the Lord’s Supper”, in which he drew on the Belhar document.
That was so good.
This Letham’s book sounds pretty interesting. I have The Holy Trinity by Letham and I love how he surveyed the triune God from many angles in that one.
I have not read that one although I hear great things. This one is excellent. It’s slim but jam-packed with biblical, historical, and theological development. He really shows how the theme of union with Christ touches every subject matter in theology. I can’t recommend it highly enough.
By the way, thanks for your great blog. Your reformedishm is so informative and edifying to read even for an evangelical dispensationalist Molinist like me.
I’m glad you’re blessed by it. BTW I have very strong Molinist leanings. It makes a lot of sense–I would love to do more study on it beyond the little bit of Craig and Plantinga I’ve read.