The exact phrase “in Christ” is used 76 times in the KJV. Here are the first usages of that term, in order.

Acts 24:24 English Standard Version (ESV) After some days Felix came with his wife Drusilla, who was Jewish, and he sent for Paul and heard him speak about faith in Christ Jesus.

Romans 3:24 English Standard Version (ESV) and are justified by his grace as a gift, through the redemption that is in Christ Jesus,

Romans 8:1 English Standard Version (ESV) There is therefore now no condemnation for those who are in Christ Jesus.^[a]

Footnote: Some manuscripts add who walk not according to the flesh (but according to the Spirit)

Romans 8:2 English Standard Version (ESV) For the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death.

Romans 8:39 English Standard Version (ESV) nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.

Romans 9:1 English Standard Version (ESV) I am speaking the truth in Christ—I am not lying; my conscience bears me witness in the Holy Spirit—

Romans 12:5 English Standard Version (ESV) so we, though many, are one body in Christ, and individually members one of another.

Romans 16:3 English Standard Version (ESV) Greet Prisca and Aquila, my fellow workers in Christ Jesus,

Scripture tells us what it means to be in Christ.

I will add only one thing that I noticed. All are personal, we are individuals in Christ, until Romans 12:5 and 16:3 which tells us we are not “in Christ” alone.