Is one who imagines himself ruling a Muslim to be an unbeliever (kafir), and moves his lips without being certain of uttering the words, considered to have committed an act of disbelief (kufr)?

Disbelief does not occur through inner thoughts or by doubting one's utterance of it. Accusing a Muslim by saying, "O disbeliever," does not lead to apostasy according to the scholars. The Hadith reported concerning this is interpreted as referring to minor disbelief, because it likens accusing a Muslim of disbelief to killing him, and killing is not an act of disbelief that expels one from the fold of Islam. A believer remains a brother to another believer even after being verbally accused of disbelief.