In the technical theological sense, genuine Islam and genuine faith are inseparable: one cannot be fully submitted to God while rejecting the truth of the message, nor genuinely affirm it while rejecting submission as such.
This does not mean observers can read hearts. A person may be treated as Muslim under the outward rules because they profess Islam, while the inward reality remains known to God. Outward legal status and inward truth must not be confused.
The distinction is essential when assessing a particular individual's religious status: visible behavior supports legal judgments but does not authorize confident claims about hidden intentions.