Is the initial failure to declare as an unbeliever someone who reviles God—before certainty is achieved—tantamount to not declaring an unbeliever an unbeliever?

We advise keeping away from the whispers of takfir (declaring someone an apostate). Cursing Allah is a grave disbelief by the consensus of Muslims. Whoever hears Allah being cursed must rebuke the one who utters it, by enjoining good and forbidding evil. The lowest degree of denunciation is denunciation by the heart, the control of which is to detest the evil and depart from its place.