CLAIM: Paul claims that non-believing people can be “sanctified.” This term always (?) refers to Christians. What does he mean by this?
RESPONSE: Paul doesn’t mean that unbelievers are made righteous (or “holy”) simply because they are married to a Christian. Such a concept is not only completely foreign to Paul’s thinking, but it also contradicts verse 16 (“For how do you know, O wife, whether you will save your husband?”).
Instead, the Greek word for “sanctified” (hagiazō) literally means “to be set apart.” Paul is saying that God will bring extra conviction and attention on a household with only one believer in it. The example of the Philippian jailor is a good example of this. This one man came to Christ, and he led his entire family to Christ (Acts 16:30ff).