UNION WITH CHRIST (and the related subject of election) is where Christianity centres.
For my use and I hope for your benefit, these have helped me think myself clear...
Peter Leithart - Chosen in Christ: Trinity & Election: To consider election, then, is simply to consider Christ. To trust in Christ is to discover God's choosing. Christ is the "mirror" of election. To be in Christ is to be in the Elect One, chosen in the Beloved by the Beloved.
Mike Reeves: Union with Christ at Transformission 2009 - three mp3s.
Rich Owen - Genesis 27: We tend to think that election is all about us, am I elect? I don’t know. And that is where the fear comes from. But its not about us, it is about Christ. Scripture wants you to know that Christ has been chosen, Christ is has been elected to be the champion of salvation.
Tim Challies reads John Murray's Redemption Accomplished and Applied: Nothing is more central or basic than union and communion with Christ. ... Union with Christ is really the central truth of the whole doctrine of salvation not only in its application but also in its once-for-all accomplishment in the finished work of Christ
Bobby Grow - The incredulity of classic calvinism and arminianism: The Gospel is “God’s life in action!” If you think “keeping-the-Law” is the barometer of salvation; think again! It is Christ’s life, it is God’s life which is the Gospel; if you’re in Him, then you are completely saved!
Blogposts here on union with Christ. And Loads more here from Phil Gons.