Ruling on saying, “So and so is going to cause us to become disbelievers”
Question

One of my classmates in college said, a few months ago, “He is going to cause us to become disbelievers,” meaning that the professor is so harsh that he will make us lose our faith and become disbelievers [this is a colloquial Arabic expression]. Has he become a disbeliever because of saying that?
Praise be to Allah.If a person says “So and so is going to cause us to become disbelievers”, what he means is that that person will push us to do that because of his bad attitude or raising doubts and the like.
That person does not become a disbeliever because of saying that, because he is speaking about something in the future that has not happened.
But saying such a thing is abhorrent in all circumstances, because disbelief (kufr) is not a matter to be taken so lightly that a person could fall into it because of some annoyance he is faced with. Using this phrase is indicative of a lack of religious commitment, and demonstrates that the issue of falling into disbelief is something that the speaker takes lightly.
What we must do is be wary of slips of the tongue. Allah, may He be exalted, says (interpretation of the meaning):
{Man does not utter any word except that with him is an observer prepared [to record]} [Qaf 50:18].
And the Prophet (blessings and peace of Allah be upon him) said: “A person may say a word which displeases Allah, thinking little of it, for which he will be thrown down into Hell.” Narrated by al-Bukhari (6478) and Muslim (2988).
And Allah knows best.