What does it mean tobelieve in Allah?

To believe in Allah means to believe in total perfection, and uniqueness.
It means to believe in His prophets because they are His, and He sent
them. We can not pick and choose. A Muslim must believe in all the
prophets of Allah.
It means to believe in the Angels! We must love them all. We cannot
hate an Angel say, for example, Michael to whom rain and the
vegetation of the Earthare attributed or Gabriel towhom revelation
(bringingdown the scriptures to the prophets) is attributed.
It means to believe in Allah's predestination of good and bad.
It means to believe in the last day: The day of Judgment.