We know that Allah has said that some of the companions of the Prophet had gone astray. Based on this, if a person were to doubt the religious commitment of a Sahabah, such as if he were to doubt if that if a specific member of the Sahabah were Muslim or not would that person be called a kaafir?
Narrated ‘Abdullah: The Prophet said, “I am your predecessor at the Lake-Fount”. ‘Abdullah added: The Prophet said, “I am your predecessor at the Lake-Fount, and some of you will be brought in front of me till I will see them and then they will be taken away from me and I will say, ‘O Lord, my companions!’ It will be said, ‘you do not know what they did after you had left.’
Narrated ‘Abdullah: The Prophet said, “I am your predecessor at the Lake-Fount”. ‘Abdullah added: The Prophet said, “I am your predecessor at the Lake-Fount, and some of you will be brought in front of me till I will see them and then they will be taken away from me and I will say, ‘O Lord, my companions!’ It will be said, ‘you do not know what they did after you had left.’