This is a question for which many answers can be heard. One person believes "in Christ" means one thing, while another believes "in Christ" means something entirely different. Where do we go to find the answer? The same place we go to find the answer to all of life's questions. We go to the Bible.
The Bible is meant to answer questions such as these according to 2 Timothy 3:16. Often with this verse, we focus on the fact that the Bible is inspired and skip past what we do with the inspired Word of God. It is for doctrine, reproof, correction, and instruction in righteousness according to the passage mentioned above.
With this in mind, what does it mean to be in Christ? In Romans 6:3-4, we see a relationship between baptism and being in Christ. We see the same relationship in Galatians 3:27 where a person is baptized into Christ. It is also interesting, given the comparison made in 1 Peter 3:20 & 21 where Noah and those saved through water in the ark, are compared to those saved when baptized into Christ.
Where do you want to be? Where is it that your soul is secure? I believe that place to be in Christ. We are saved by the grace of God through faith according to Ephesians 2:8. Without grace we cannot be saved. Who receives God's grace? Those who are in Christ receive his grace. After examining these verses, there is a very important question that all need to ask. Am I in Christ?