Blessed are you when people insult you, persecute you and falsely say all kinds of evil against you because of me. (Matt 5:11)It's true, Stephen was martyred because he was a Christian. Paul was abused because he was a Christian. James was slaughtered because he was a Christian. But what if you're being mistreated, but not because you're a Christian?
First, "because you're a Christian" is a vague phrase. What counts as being mistreated "because you're a Christian"? Perhaps you're being mistreated "because you're a Christian" when:
(a) The person(s) mistreating you believes that you're a Christian
(b) The person(s) mistreating you harbors ill-will against Christianity
(c) The person(s) mistreating you is motivated by that ill-will when
This seems a clear cut case of being mistreated "because you're a Christian," but these criteria are entirely too strict. Herod's execution of James (Acts 12) doesn't fit this criteria at all. Herod arguably didn't harbor ill-will toward Christianity nor was his act motivated by it nor was it even necessarily the case that he believed James was a Christian. Herod wanted the approval of the Jews and they believed James was a Christian and harbored ill-will toward Christianity. James was mistreated "because he was a Christian," but it's quite possible that the person mistreating him didn't meet any of the above criteria.
This opens the question: whose perspective counts? Is "because-you're-a-Christian" mistreatment about what the perpetrator thinks about the situation? Is it about what onlookers think about the situation? Is it about what you think about the situation? Is it about what God thinks about the situation?
i'm not sure i have an answer to what exactly is required for mistreatment to be "because you're a Christian," but i will offer two things i think point us in the right direction.
(1) i do certain things because i'm a Christian, and it may be the case that no one around me knows why i'm doing those things. A co-worker may despise me because he thinks i'm a "goody-two-shoes" and may even give me a hard time because of it. But he's not aware that the particular reason i perform "goody-two-shoes"-types of behavior is because i'm a Christian, and he would just as soon give me a hard time either way. i may have to stand up for what's right because i'm a Christian, and people may oppose me for standing up for what's right even though they aren't consciously opposing me for being a Christian. Yet i believe this still clearly counts as a case of being mistreated "because i'm a Christian."
(2) God expects that everything i do is to be done in the name of the Lord (Col 3:17) and to His glory (1Cor 10:31). That doesn't mean that people who don't like the way i blow my nose are persecuting me. But it means there is never a time when i'm not Christ's representative. There's no "off-duty hours" for a Christian. If i am a Christian, i am obligated to be Christian in all situations, times, and circumstances i face. Thus, just because people mistreating me may not know or care whether i'm a Christian, i'm nonetheless obligated to behave as a Christian in response to their mistreatment.
Second, is it really the case that the NT condemnation of violent retaliation only applies when i'm being mistreated because i'm a Christian?
Jesus' instructions in the Sermon on the Mount regarding turning the other cheek and loving your enemies include no such qualification about the reason for mistreatment. It's arguable that the Roman soldiers imposing their load upon people for a mile at a time didn't care about the particular beliefs that person held (Matt 5:41). Rather, Jesus' rationale is that you should do good to those who do evil to you because God blesses even the wicked (Matt 5:45).
Peter's instructions about not returning evil for evil do not have a mistreatment-because-of-being-a-Christian qualification. Peter, rather, is giving instructions first about how Christians are to treat each other (1Pet 3:8), and then as he continues, he develops into the topic of Christian persecution (1Pet 3:13f).
Paul's instructions (Rom 12:14-21) do not contain any qualification about the motive for mistreatment. And Paul, too, includes how Christians treat one another in his instructions about retaliation (Rom 12:16).
Therefore, the instructions given to Christians which condemn vengeance/retaliation/turning the other cheek are not restricted to circumstances in which the mistreatment is somehow motivated against Christianity. Rather, these instructions are given to disciples as general and universal ethical norms.