There is a Muslim guy whom I know who said really bad things about Islam to someone, presumably pretending to be an atheist. He said really bad things, like "Your God wants you to do His dirty work;" and that the Madrassahs are universities of ignorance and that "your Prophet" must be held responsible for this (we seek refuge from Allaah from such speech). Later on, he said that he was just checking that guy's faith and how long he could defend it. I found this really confusing because in his later messages he was thanking Allaah. I was not sure if he has an excuse of misinterpretation. So I said that I would consider this Kufr (disbelief) unless I am proven otherwise, but even though I said that, I still had that feeling of 'What if I am missing something, and maybe he means something else'. Whenever I try to think that I will tell him that this is Kufr and that he repeat his testimony of faith, I feel afraid that I might be wrong even though I said that I would consider it Kufr! Besides, someone told me that it is not Kufr until the truth is made clear to him and he still does not admit it. Is this misinterpretation, Waswaas (devilish whisperings), or doubt (the kind that make a person a disbeliever if he doubts the Kufr of someone else)? I think that the devil was casting these doubts in my heart, and I am really really confused nowadays.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

The scholars stated that both joking and seriousness in uttering words of Kufr (disbelief) are the same and that there is no difference between them.

Ibn Al-Qayyim said in I‘laam Al-Muwaqqi‘een’:

“Allah and His Messenger did not make joking an excuse that exempts a person from sin; rather, the person who says a word of disbelief is more deserving of punishment. Do you not see that Allah has excused the person who is forced to say the word of Kufr if his heart is reassured with faith while Allah did not excuse the person who is joking; Allah says (what means): {And if you ask them, they will surely say, “We were only conversing and playing.” Say: Is it Allah and His verses and His Messenger that you were mocking?} [Quran 9:65]"

Therefore, what that man has done is very serious; saying that he is trying to test the other person is not an excuse for him because it is not with such obnoxious words that we test people.

If he thought that acting in that manner was permissible, then we hope that he would be excused because he misinterpreted the matter. It should be noted that there is a difference between the statement and the person who said it, and the statement may be Kufr while its sayer does not become a Kaafir (disbeliever), as we have already clarified in fatwas 8106 and 87963.

The issue of 'whoever does not declare a Kaafir to be a Kaafir becomes a Kaafir himself' is related to someone whose Kufr is confirmed with certainty.

We have already warned you in our previous fatwas against the danger of Waswaas (obsessive whispering) in regard to declaring someone to be a Kaafir and had clarified that you must be careful about it because of its evil consequences. So we emphasize here again that you should be cautious about it if you wish to be safe from this tribulation.

Allah knows best.