بِاللَّهِ refers to belief in Allah, the One worthy of worship. مَلَائِكَتِهِ refers to His angels, كُتُبِهِ to His revealed books, and رُسُلِهِ to the messengers He sent.
مَلَائِكَة، كُتُب، رُسُل are plurals, each linked to Allah by the possessive suffix. They should not be confused: angels, scriptures, and messengers are distinct categories.
Respect for the Qur'an, receiving authentic guidance of messengers seriously, and striving to live by divine guidance are practical consequences of such belief.