Chastity is the resolve to keep a heart pure and on fire for Jesus. It is not just the virtue that protects us from sexual sin. Chastity is the watchman that guards the heart from any passion that would douse or misdirect the love that belongs to the bridegroom himself.
In I Corinthians 6 Paul is dealing with sexual immorality. In giving pastoral instruction, he says something that is, at first, difficult to understand. He says,
‘Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a prostitute? Never! Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, ‘The two will become one flesh.’ But he who is joined to the Lord becomes one spirit with him’ (vs. 15-17).
Now what is strange about this teaching is that Paul seems to suggest that the spirit of a Christian is wedded to the spirit of Christ. The problem of prostitution is not just that it violates a command of God or that it infringes upon the covenant of a human marriage. The depth of the sin is nothing other than infidelity against Christ himself.