What is the ruling on the oath of allegiance sworn by a Muslim man to a group, committing absolutely and without reservation to work for the service of the religion, then he was imprisoned and sentenced for a period, then he was released while the group remains? Does this oath still bind him despite his imprisonment, and can he be released from it?

Serving the religion and supporting its Sacred Law is an obligation upon every Muslim to the best of their ability, and it is not a condition that this be through a specific group. The fundamental obligation is obedience to Allah and His Messenger, and then obedience to those in authority accordingly. Taking an oath of allegiance (bay'ah) for other matters does not necessitate obligation, for it is the covenant with Allah Almighty that must be fulfilled, and it suffices for all else, encompassing cooperation in righteousness and piety. It is not permissible for anyone to take an oath of allegiance to befriend whoever they befriend and to antagonize whoever they antagonize. Rather, the true covenant is obedience to Allah and His Messenger and cooperation in truth, righteousness, and piety, not in sin and aggression.

Therefore, the questioner is not bound by this oath of allegiance except by what Islam obliges him to do: cooperation in righteousness and piety and mutual support for the truth. We should note that the oath of allegiance mentioned in the hadiths is for the general Imam whom people unite upon, and the majority of scholars believe that fulfilling a promise is recommended, not obligatory.