What is the ruling on a Muslim who pretends to embrace another religion in front of the enemies of Islam to expose their conspiracies and destroy their plans, and then declares his Islam and invites them to it?

A Muslim is not permitted to utter words of disbelief (kufr), or to show allegiance to disbelievers and join their parties, except in cases of necessity, such as fear of death, or if he is instructed to do so by the Imam of the Muslims to achieve a benefit for the Ummah. It is not permissible to undertake this action on one's own initiative.