Who you are shapes how you act. How you act shapes who you are. Identity and behavior are mutually reinforcing.

For example, a generous person gives liberally to those in need. But how did she become generous in the first place? By giving liberally. An alcoholic drinks too much alcohol. How did he become an alcoholic? By drinking too much in the first place. A loving married couple takes delight in serving one another. How did they become so loving? By serving one another.

To become the people God created us to be, Christ saved us to be, and the Spirit empowers us to be, we must pay attention to who we are and how we act.

In 1 Corinthians 6:12-20, Paul rebukes the Corinthians—some of them, anyway—for consorting with prostitutes. In verses 12-14, he counters their false theology of freedom. In verses 15-17, he points out the crucial connection between who we are and how we act.

Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, “The two will become one flesh.”  But he who unites himself with the Lord is one with him in spirit.

Paul describes Christian identity in terms of spiritual union with Christ. “Your bodies,” he writes, “are members of Christ himself.” Further, “he who unites himself with the Lord is one with him in spirit.” One of the most common expressions in Paul’s letters is the two-word phrase, “in Christ.” If you are in Christ—that is, if you are one with him in spirit—you will act in certain ways but not others because Christ would act in those ways but not others. Obviously, Christ would not consort with prostitutes. Ergo, if you are in Christ, you will not either. Your identity in Christ shapes your behavior.

Unfortunately, the Corinthians’ behavior was deforming their identity. “Do you not know,” Paul asks, “that he who unites himself with a prostitute is one with her in body?” God created sex to weave the physical, emotional, and spiritual lives of two human beings into a harmonious whole. “The two shall become one flesh” (Genesis 2:24). Rather than being formed in the character of Christ or into the harmonious whole of a Christian marriage, the Corinthians’ sexual immorality was forming them in the character of a prostitute.

We live in a day and age of “casual sex” and “hooking up” without commitment. In Christ, there’s no such thing as casual and commitment-less sex. Christ himself remained celibate, but he blessed marriage for others. If our character is Christ’s, then our rule is fidelity within marriage and chastity without it. To the extent that we, like the Corinthians, don’t follow this rule, we need to repent.

Who you are shapes how you act, and vice versa.