Question: What is meant by the statement "Approval of kufr is kufr"? How does a person approve kufr?
ANSWER
Approval of kufr (disbelief) means wishing that a certain Muslim became a disbeliever (kafir) or wishing that he died a disbeliever or liking the disbelief of a disbeliever. Wishing that a disbeliever remained in disbelief is not approval of kufr. (Fatawa al-Haramain)

Let me give some examples to illustrate the subject:
1. If a Muslim wears a cross, telling him to continue to wear it is an act of approval of his disbelief.

2. Praying for a certain Muslim, "O my Lord, make him die a disbeliever," means approving of his disbelief.

3. He who calls a sinful Muslim a disbeliever becomes a disbeliever himself. It is declared in a hadith-i sharif, "Do not call a Muslim a disbeliever because of the sins he commits. He who calls him a disbeliever will become one himself" (Bukhari). A Muslim who says "La ilaha ill-Allah" is a Muslim who believes in the six fundamentals expressed in the (formula of faith called) Amantu. It is mentioned in other hadith-i sharifs that one has to believe in the six fundamentals expressed in the Amantu so as to be considered a Muslim. Of these six fundamentals, anyone who does not believe in any of them is not a Muslim. For example, if he disbelieves in qadar or one of the prophets, he becomes a disbeliever. Jews are disbelievers (kafirs) because they do not believe in Hadrat 'Isa and Muhammad 'alaihis-salam. Christians, too, are disbelievers because they disbelieve in our Prophet.

4. If one's son or daughter becomes a disbeliever, it is an act of approval of kufr not to be angry with him/her, not to stop him/her, or to receive the news tolerantly.

5. Giving one's consent to the marriage of one's daughter to a non-Muslim means approval of kufr.

6. When a disbeliever wants to become a Muslim and needs you, it causes disbelief to delay his becoming a Muslim on a plea of some work, that is, to give consent to his remaining in disbelief.

Calling a Muslim a disbeliever
Question: It is said, "Anyone who calls a Muslim a disbeliever becomes one himself." If we call a Muslim a disbeliever for the sheer purpose of insulting him, is it considered an act of disbelief, too?
ANSWER
Calling a Muslim a disbeliever to show his evil character will not cause disbelief. It would cause disbelief if calling him so was intended to express one's wish that that Muslim were a disbeliever. (Ethics of Islam)