Criteria For Takfir Or Calling Others Disbelievers
The one who denies the divine message despite realizing the inherent truth in it or does not bother to investigate the truth despite having the means to knowledge are disbelievers. and those who are inclined to accept the truth whatever it is, are potentially Muslims by nature, even if they may or may not receive the message of Islam.
It is for God to decide after considering all the circumstances of the person and the knowledge he had whether he was an innate believer or not. We do not have this authority to put a disbeliever tag on others. We can call a person non-muslim if he is claiming to be not a Muslim.