What does it mean to believe in Allah?
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 Earth are attributed or Gabriel to whom revelation (bringing down 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.