A Muslim is defined as one who has submitted to the idea that there is a God, that there is only one God. That Muhammad was this God's final prophet.

That's it. If one believes in that, even if that person never tells another human of this fact, the person is a Muslim. If a person ceases to believe that, even in private, that person is no longer a Muslim.

Nothing else exactly removes one from the religion. The issue I think others on the outside thinks about Islam is that every Muslim is guaranteed heaven. They're not.

A Muslim who commits multitude of sins and is unrepentant can go to hell.

Drinking alcohol and fornication is strictly prohibited in Islam, I have Muslim friends who drink and fornicate like it's the last days of the Roman Empire. At no point do I have any standing to not consider themselves Muslim.

There is also no centralised Church who can define one as Muslim or not. Even though a number of Muslims seem to think they can, there is no religious rule or ideal where one Muslim can claim another to be a non-Muslim. Not unless the person stopped submitting to that one belief mentioned above.

Of course one is required to obey the rules, but that's God who knows what rules are and are not being followed. He forgives and He punishes, as He wills.

If the idea that every rule of Islam must be 100% followed and any error makes one a non-Muslim, then there's never been a Muslim person ever.

For e.g. The Prophet was reported to have said “The Jews split into seventy-one sects, one of which will be in Paradise and seventy in Hell. The Christians split into seventy-two sects, seventy-one of which will be in Hell and one in Paradise. I swear by the One Whose Hand is the soul of Muhammad, my nation will split into seventy-three sects, one of which will be in Paradise and seventy-two in Hell.” It was said: “O Messenger of Allah, who are they?” He said: “The main body.”

Even though they're considered members of his faith, he still says they will go to hell.