She raised a knife against him and threatened him so he would divorce her. Does the divorce count as such?

If you were afraid that your wife would carry out her threat to attack you with the knife that she had with her, then this comes under the heading of being forced, and the divorce does not count as such. Ibn al-Qayyim (may Allaah have mercy on him) said: The Sahaabah issued fatwas saying that a divorce issued under compulsion does not count as such. It is narrated in a saheeh report from ‘Umar that a man suspended himself from a mountain by a rope to collect honey and his wife came and said: Either I cut the rope or your divorce me. He adjured her by Allaah and she insisted so he divorced her. He came to ‘Umar and told him about that, and he said to him: Go back to your wife, for this is not a divorce. The ruling that it does not count as a divorce was also narrated from ‘Ali, Ibn ‘Umar and Ibn al-Zubayr (may Allaah be pleased with them). End quote from Zaad al-Ma’aad (5/208). 

And he (may Allaah have mercy on him) said: Imam Ahmad said in the report of Abu’l-Haarith: If a man who is forced issues a divorce, the divorce is not binding; if what was done to Thaabit ibn al-Ahnaf is done to him, then he is forced, because they squeezed Thaabit’s foot until he divorced his wife, then he went to Ibn ‘Umar and Ibn al-Zubayr, and they did not think it counted as anything, as Allaah says (interpretation of the meaning): “except him who is forced thereto and whose heart is at rest with Faith” [al-Nahl 16:106]. Imam al-Shaafa’i (may Allaah have mercy on him) quoted this verse as evidence that the divorce issued by one who is forced does not count as such. 

In Sunan Ibn Majaah it is narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has pardoned my ummah for mistakes, what they forget and what they are forced to do.” End quote from I’laam al-Muwaqqi’een (4/51). 

In al-Ikhtiyaaraat by Shaykh al-Islam it says: The divorce issued by one who is forced does not count. Force or compulsion happens by means of an explicit threat or when he thinks it most likely that he or his property will be harmed even if there is no explicit threat. He said elsewhere: The idea that the threat should be thought mot likely to be carried out is not correct, rather the correct view is that the likelihood of the threat being carried out or not should be equal; then it is a case of compulsion. End quote from al-Fataawa al-Kubra (5/568). 

But if you knew that your wife would never harm you, or you could put a stop to her threat without causing harm, then you were not forced in that case, and your divorce counts as such. 

What we have mentioned here is the ruling and guidelines on this issue. As for the ruling in your case, the details must be examined further, hence we say that you have to refer to the shar’i court or to a trustworthy scholar in your own land, so that he can hear from both of you and determine whether this was a case of force or not. 

And Allaah knows best.