Question: What does "contagion of disbelief" mean? Does a person who commits something that has been reported to cause disbelief not become a disbeliever?
ANSWER
Hadrat Imam-i Rabbani says the following about this issue:

Remaining eternally in Hell is the retribution for disbelief (kufr). Our scholars say that if a person, though having faith (iman), values the rites and customs of disbelievers, he will lose his faith. Most of today's Muslims have caught this nuisance. On the other hand, our Prophet said, "He who has a mote of faith in his heart will not remain in Hell eternally but will be taken out." The explanation of this is as follows:

If a person disbelieves, doubts about, or disapproves even one of the Islamic tenets that are to be believed, his faith goes away; he becomes a disbeliever (kafir). He will be burned eternally in Hell. If a person expresses the word tawhid, believes its meaning, says that Muhammad 'alaihis-salam is Allah’s Prophet and that whatever he has said is true and good, and believes that those who disagree with him are wrong and evil, and takes his last breath in this faith and goes to the other world with faith, he will be treated differently. If he, on the days and nights which disbelievers hold sacred, does as they do or if he commits a sin by slighting or ignoring its importance, these acts are called "contagion of disbelief." This person, too, will go to Hell, but he will not remain therein eternally because he has a mote of faith in his heart.

As for those Believers who have grave sins [and have died without making repentance], Allahu ta'ala will forgive those sins if He wills. Or He will torment them in Hell until they are purified of their sins, if He wills. Hell fire, whether endless or temporary, is for disbelief and for the symptoms and contagion of disbelief. The grave sins committed by those Believers who shun disbelief will be forgiven either through Allahu ta’ala’s compassion for the sake of their faith or by their repenting with their hearts and seeking forgiveness with their tongues and doing a virtuous deed with their bodies or through being blessed with shafa’at (intercession). If a sin involves violation of others' rights as well, it is necessary to get the wronged person's forgiveness. Those who are not forgiven by such means will be purified through worldly problems and cares or through the throes which they will suffer while dying. If they are not purified with these either, some of them will attain forgiveness by suffering torment in their graves. And others will be forgiven through the torment and suffering in the grave in addition to the vehemence of the Day of Resurrection; they, thereby, being without sins, there will be no need for them to be purified through Hell’s torment. As a matter of fact, it is declared in the 82nd verse of Surah al-An'am, “Those who have faith and who do not smear their faith with polytheism are secured against remaining eternally in Hell. There is not such fear for them,” which proves our word to be correct. For, in this verse, zulm means shirk (polytheism).

One day this faqir [Hadrat Imam-i Rabbani means himself] visited a sick person. He was about to die. I paid tawajjuh toward his heart. His heart had darkened. I did my best to clear away the darkness; it was of no avail. Meditating deeply for a long while, I came to realize that that blackness was the contagion and symptom of disbelief, and that it was caused by his relationships and friendliness with disbelievers and disbelief. Try hard as I would, that darkness could not be cleared away. It was then realized that that evil would be cleared away only with the fire of Hell, the punishment for disbelief. Yet, since a tiny light of faith was seen in his heart, owing to this he will be taken out of Hell. Having seen the sick person in that state, I went deep into meditating whether I should perform salat al-janazah for him. After searching my heart for a long time, I realized that it would be necessary to perform it. This means that we should perform salat al-janazah for those who, though having faith in their hearts, are intimate with disbelievers [although there is no extreme necessity to do so] and follow their ceremonies and Easters. We should not look on them as disbelievers. We should believe that owing to their faith these people will be taken out of Hell. But there is no pardon or forgiveness for those who do not have faith, [for those who do not like even one word or habit of Muhammad ‘alaihis-salam]; in return for their disbelief they will remain in Hell’s torment eternally. (Vol. 1, Letter 266)

Question: If a pious Muslim does deeds that involve contagion of disbelief, such as backbiting someone or committing any other sin without becoming sad for committing it, will he remain in Hell eternally?
ANSWER
He, unlike disbelievers, will not remain in Hell eternally. Which person who is backbiting someone else feels sad while he is backbiting? But the truth is that committing a sin without feeling sad for committing it causes disbelief. Anyone who utters words or does deeds that are symptomatic of disbelief becomes a disbeliever by judgment. Is a person who backbites someone called a disbeliever? If he, when asked, believes that backbiting is haram, he is not called a disbeliever. If he were called a disbeliever, there would be no Muslim on earth. Our scholars state that those who commit contagion of disbelief will not remain in Hell eternally unlike disbelievers.