Ibn Qayyim (rahimahullaah) said:
“Hypocrisy is a deep rooted and all-pervading spiritual disease.
A person’s heart could be overflowing with it yet be oblivious of it due to its hidden and subtle nature;
frequently does it lead a person to think he is acting rightly when in reality he is spreading corruption.
It is of two types, major and minor: major hypocrisy leads to eternal punishment in the lowest depth of Hell.
It is to actually display faith in Allaah, His Angels, and His books, His Messengers and the Last Day, whereas inwardly one is devoid of such belief – indeed disbelieving in it.
He does not believe that Allaah spoke to a man amongst men who he appointed to be a Messenger, guiding them by His provision and warning them of His punishment.
Allaah, the Exalted, says,
“There is a sickness in their hearts and Allaah has increased their sickness; they will have a painful punishment on account of their lies.”
“The sickness here is the sickness of doubts and desires; the heart is beset by two diseases that eject it from its state of balance and normalcy: the disease of false doubts and the disease of vile desires. Disbelief, hypocrisy, doubts, and innovations all arise from the disease of doubts. Fornication, the love of lewd deeds, and sins all arise from the disease of desires. This verse also shows that it is as a consequence of over sins that Allaah punishes him by by afflicting him with more sins.”
At-Tabaaree gives an example of how this verse was actualised by stating that each time a Soorah was revealed of the Qur’aan containing new injunctions, this increased them in their doubts and hence their disease increased.
“Each time a Soorah is revealed there are some among them who say, ‘Which of you has this increased in faith? As for those who believe, it increases their faith and they rejoice at it. But as for those with sickness in their hearts, it adds defilement to their defilement and they die as disbelievers.’”
Ibn Qayyim went on to say:
“Whoever falls prey to the claws of their doubts will have his faith shredded to pieces;
whoever allows his heart to be open to their vile tribulations will find himself a burning furnace;
and whoever lends an ear to their deceptions will find them coming between him and firm belief.
Indeed, the corruption they cause on earth is great but most people are unaware.
“And when it is said to them, ‘Do not cause corruption on the earth,’ they say, ‘We are but reformers.’ Unquestionably, it is they who are the corrupters, but they perceive [it] not.”
Excerpts from: Characteristics of the Hypocrites “Sifat al-Munaffiqeen” (pp. 22-23), by Ibn Qayyim al-Jawziyyah (rahimahullaah)
 Allaah, the Exalted is He, says,
“The hypocrites are in the lowest level of Hell and you will not find anyone to help them.”
Regarding the Hypocrites
Jihaad Against the Hypocrites: The Danger of Hypocrisy and Major vs. Minor Hypocrisy