“Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.” (2 Corinthians 5:17)
I’m indebted to E. Stanley Jones, a missionary who served in India for 50 years, and his superb daily devotional, In Christ, for showing me the importance of this phrase. I highly recommend his book which highlights the use of the phrase “in Christ” throughout the New Testament.
According to Dr. Jones, when we think about being “in Christ” we should realize that Paul was not talking about being in religion. Few people have been more into religion than Paul before he met Jesus. Paul was so religious he fervently persecuted followers of Jesus, sure that he was pleasing God by trying to snuff them out.
It is possible to be in religion, but not be in Christ. It is possible to be in church, and not be in Christ. We can be in doctrine, or theology, and not be in Christ. We can be in ministry and not be in Christ. We can be committed to Christ, and believe a lot of things about Christ, and still not be in Christ.
To be in Christ locates us in a Person, right now.
Unless we are “in Christ” it’s like we have a powerful engine in our automobile but we cannot find our ignition key that turns the engine on. Being ‘in Christ’ is the ignition key, opening us up to experience “all spiritual blessings in Heavenly places.” (Ephesians 1:3)
Paul essentially writes: I live because Christ lives in me and I live in Christ.
Just as you sometimes cannot find the keys to your automobile, have you misplaced this critical spiritual key – are you living in Christ?
Dick Woodward, 09 October 2013
4SpiritualSecrets 
I believe it means to be consumed by Christ so that His spirit is controlling you.