In addition from the comment section, Allah states inside the Qur'an that the concept of Heaven is to accept Islam after being given a clear explanation. This is based on the word of Him from surah al-Baqarah, verses 111 and 112. In verse 111, Allah declares that Jews and Christians claim that no one can enter Heaven unless they are Jews and Christians. Allah states that it is only their assumption. In fact, He challenges them to come up with arguments, if they believe they are on the right path.
In verse 112, God also explains who actually goes into heaven, those who "as-la-ma" (surrender) themselves to Him.
From this understanding, it is clear that those who will be going to the Heaven/paradise is those who;
- Explanation of knowledge in the form on strong argument
- After the strong explanation, the person accept and surrender to Him.
Those who reject these two factor are then being called non-believer (kafir). However, if there is no direct explanation, or there is an explanation but is not strong, or gives a wrong explanation, or the Muslims themselves display a false character in the meaning of Islam, then the rejection cannot be considered as non-believer (kafir).