If Muslims don't believe in Allah are they still a Muslim?

Prayer is one of the five pillars of Islam. If one doesn't pray, you have to ask another question. Is this person not praying out of disbelief or laziness? If one doesn't pray because they don't believe prayer is necessary, then they are not considered Muslim. If one doesn't pray because they are lazy, they are considered sinners. However, they are not out of the realm of Islam.

Any sin can be forgiven by God with the exception of disbelief.