Complex sentence: Shia Islam primarily contrasts with Sunni Islam, whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr (who was appointed Caliph through a Shura, i.e. consensus) to be the correct Caliph.
Split into simpler sentences: Shia Islam primarily contrasts with Sunni Islam, whose adherents believe that Muhammad did not appoint a successor. Instead they consider Abu Bakr (who was appointed Caliph through a Shura, i.e. consensus) to be the correct Caliph.