Whoever says 'lā ilāha illa Allah (there is no god but Allah)' and disbelieves in everything worshiped besides Allah, his property and blood becomes inviolable, and his reckoning will be with Allah

The Prophet (may Allah's peace and blessings be upon him) informs that if a person says and bears witness that "there is no god but Allah" - which means that none is truly worthy of worship except Allah - and disbelieves in everything worshiped apart from Allah and disavows all other religions besides Islam, his property and blood become inviolable to the Muslims. We only judge according to his apparent reality.

So, his property may not be taken, or his blood be shed unless he commits a crime or an offence that entails such punishment as per Islamic laws. And Allah will be responsible for his reckoning on the Day of Judgment. If he is sincere, He will reward him, and if he is insincere, He will punish him.