Is a Muslim who commits an act necessitating apostasy, and then pronounces the two declarations of faith (Shahadatain) during prayer, without the intention of returning to Islam, considered a Muslim by that act?

Reviling the religion of Islam is an act of apostasy that expels one from the fold of Islam if the reviler intends to insult the religion itself. It is a sin and an act of wickedness if the reviler intends to insult the person with whom they are disputing.

Whoever reviles the religion of Islam but does not renew their repentance, yet testifies to faith, prays, and fasts, is judged to be Muslim, based on the Prophet, peace and blessings be upon him, who said: "I have been commanded to fight the people until they say: 'There is no god but Allah.' If they say it, then their blood and wealth are protected from me, except by its right, and their reckoning is with Allah, the Almighty, the Majestic." This establishes the Islam of both the original disbeliever and the apostate.

As for repentance between a servant and their Lord, it must involve sincerity to Allah, regret for the action, abstaining from the sin, a firm resolve not to return to it, and that the repentance occurs at a time when it is accepted.