"When they speak of being 'in Christ' or of Christ being 'in them', that is not simply a way of saying that they are thinking about Christ or copying Him."

John 15:5
Creative Commons AttributionJohn 15:5 - Credit: Ian Britton
In Instruments in the Redeemer's Hands: People in Need of Change Helping People in Need of Change, Paul David Tripp writes, "[Being 'in Christ'] means that we have been fully united to him and received all he has promised us. This goes far beyond the fact that we have been forgiven and are going to heaven. Something more fundamental has happened. Paul describes it in Ephesians 2:6-7: 'And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus.' John 15 captures this identity with a metaphor, saying that as believers we are like branches attached to a vine. We can only bear fruit because of our union with Christ the Vine. We have life because we are 'in' the Vine" (p. 260).

Other passages include . . . 

2 Corinthians 5:17 (ESV): "Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come."

Ephesians 2:4-10 (ESV): "But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, 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, so that in the coming ages he might show the immeasurable riches of his grace in kindness toward us in Christ Jesus. For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them."

Colossians 1:27 (ESV): "To them God chose to make known how great among the Gentiles are the riches of the glory of this mystery, which is Christ in you, the hope of glory."

And the words of Christ in John 17:20-21 (ESV): "I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me."