What is the ruling on one who denies the greatness of the Messenger out of ignorance and misgivings, intending by "greatness" absolute power, while still maintaining the five daily prayers? Does this apostatize him from the faith?

It is advised to ignore obsessive thoughts and to occupy the mind with memorizing the Qur’an, gaining deeper understanding of the religion, and reading books of Seerah (Prophet's biography). Inner thoughts have no effect unless they are uttered. If "greatness" is negated with the intention of negating the greatness of omnipotence, dominion, and subjugation that are exclusive to Allah, then there is no blame on the one who says it. The glorification of the Messenger, peace and blessings of Allah be upon him, by Muslims—a glorification free from exaggeration—is an obligation. The texts of the Qur’an and Sunnah, and the consensus of the Ummah, have indicated the necessity of glorifying him, honoring him, and showing him respect.