In the New Testament, "in Christ" is the most common usage, demonstrating membership in the body of Christ and full agreement with his will.
"For Christ" is also used to indicate loyalty and devotion. Here are verses where it is used, sometimes directly as "for Christ", sometimes in the construction "for the sake of Christ", and sometimes in rearranged phrases that separate "for" and "Christ". I exclude verses where "for" means "because".
- 1 Cor 4:10
- 2 Cor 12:10
- Ephesians 1:12; 4:1; 5:21
- Philippians 1:13,29; 2:21,30; 3:7,8
- Colossians 4:3
- Philemon 1:6
- Hebrews 11:26
Philippians has the most usages:
As a result, it has become clear throughout the whole palace guard and
to everyone else that I am in chains for Christ. (1:13)
For it has been granted to you on behalf of Christ not only to believe
in him, but also to suffer for him (1:29)
For everyone looks out for their own interests, not those of Jesus
because he almost died for the work of Christ. He risked his life to
make up for the help you yourselves could not give me. (2:30)
But whatever were gains to me I now consider loss for the sake of
What is more, I consider everything a loss because of the surpassing
worth of knowing Christ Jesus my Lord, for whose sake I have lost all
things. I consider them garbage, that I may gain Christ (3:8)
Despite "for Christ" being the less common, the use of the construction "for the sake of Christ" in Philippians is so powerful (and a memorable verse) that I would not discourage its use.
The concept of being "in Christ" was not easy for me to grasp as a new believer. The meaning of it is richer than "for Christ", but to a general audience, "for Christ" will be more understandable.
The one caveat is that the goal of being "in" Christ is to surrender to his will, whereas being "for" Christ connotes exercising your independent will in alignment with his. The focus is more on you, less on him.