“The fornicator does not fornicate while being a believer” (al-Bukhārī).
A classical interpretation takes this to negate the perfection of faith at the time of the grave sin rather than necessarily proving that the person has irreversibly abandoned the foundational assent of faith. Expressions involving “faith” may refer to different respects in different texts.
This reading does not soften the warning. It seeks to reconcile severe admonition with other evidence that grave sin and a continuing fundamental designation of faith can coexist.