Chapter 13 is a very weighty chapter, and yet so essential to the Christian life. What is Owen talking about here in this chapter when he writes about "Communion with Christ Purchased in Grace"? Simply, he is speaking about the reality of the Christian being united to Jesus Christ. Owen writes,
there is almost nothing that Christ has done, but we are said to have done it with him (Gal. 2:20, 2 Tim. 2:11, Col.3:3, Rom. 6:4, Col. 2:12; 3:1, Eph. 2:5-6)
If you want to know what he is speaking about, check out the verse references.
Galatians 2:20: I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
2 Timothy 2:11-12: The saying is trustworthy; for: If we have died with him, we will also live with him; if we endure, we will also reign with him;
Colossians 3:3: For you have died, and your life is hidden with Christ in God.
Romans 6:4: We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in the newness of life.
Colossians 2:12, 3:1: having been buried with him in baptism in which you were also raised with him through faith in the powerful working of God, who raised him from the dead...If then you have been raised with Christ, seek the things that are above, where Christ is, seated at the right hand of God.
Ephesians 2:5-6: even when we were dead in our trespasses, (God) made us alive together with Christ-by grace you have been saved-and raised us up with him and seated us with him in the heavenly places in Christ Jesus.
Do we now understand what Owen means when he says that there is almost nothing Christ has done that we are not said to have done with him? This is union with Christ, and union with Christ is at the heart of our salvation. We are united to Christ in his life of perfect obedience (read chapter 15 if you haven't yet for a wonderful explanation on why Christ's life, not just his death and resurrection, is so essential for the Christian). We are united to Christ in his death. We are united to him in his resurrection. We are united to him in his ascension into heaven. We are united to him in his glorification. And we will be united to him in his future reign in the new heaven and new earth. This is amazing! This should just floor us. It should bring tears to our eyes to think that we, who were once enemies of God, objects of wrath, infinitely guilty before his infinite holiness, worthy of nothing but damnation, that we would be brought into such a rich inheritance because of what Jesus Christ has done for us. This is the reality of union with Christ and communion with the Triune God.