There are many different views in Islam, but all Muslims have major things in common:
- Belief in Allah as the one and only God who created the world
- Belief in Quran as the messages of Allah to the human
- Belief in prophet Muhammad (PBUH) as the last prophet sent by Allah
- Belief in Ka'ba as the unique Qibla to pray towards
- Belief in all the previous prophets including Adam, Noah, Ibrahim, Musa and Isa
- Belief in the future life, the day of judgement, heaven and hell
- And many other things
So this is my question: If there is another group of people who believe in Allah as the unique God, Prophet Muhammad (PBUH) as the last prophet, and the holy Quran, is another Muslim allowed to call them unbeliever (Kafir)? - By Kafir I mean Kufr-Ul-Kibr as stated in @DanAndrew answer below.