Muhammad bin Ishaq narrated that Ibn `Abbas said that,
﴿وَمِنَ النَّاسِ مَن يَقُولُ ءَامَنَّا بِاللَّهِ وَبِالْيَوْمِ الأْخِرِ وَمَا هُم بِمُؤْمِنِينَ ﴾
(And of mankind, there are some who say: "We believe in Allah and the Last Day'' while in fact they do not believe) "This refers to the hypocrites among the Aws and Khazraj and those who behaved as they did.''
This is how Abu Al-`Aliyah, Al-Hasan, Qatadah and As-Suddi explained this Ayah. Allah revealed the characteristics of the hypocrites, so that the believers would not be deceived by their outer appearance, thus saving the believers from a great evil. Otherwise, the believers might think that the hypocrites were believers, when in reality they are disbelievers. To consider the sinners as righteous people is extremely dangerous, Allah said,
﴿وَمِنَ النَّاسِ مَن يَقُولُ ءَامَنَّا بِاللَّهِ وَبِالْيَوْمِ الأْخِرِ وَمَا هُم بِمُؤْمِنِينَ ﴾
(And of mankind, there are some who say: "We believe in Allah and the Last Day'' while in fact they do not believe) meaning, they utter these false statements only with their tongues, just as Allah said,
﴿إِذَا جَآءَكَ الْمُنَـفِقُونَ قَالُواْ نَشْهَدُ إِنَّكَ لَرَسُولُ اللَّهِ وَاللَّهُ يَعْلَمُ إِنَّكَ لَرَسُولُهُ﴾
(When the hypocrites come to you (O Muhammad ), they say: "We bear witness that you are indeed the Messenger of Allah.'' Allah knows that you are indeed His Messenger) (63:1).
This Ayah means that the hypocrites utter these statements only when they meet you, not because they actually believe what they are saying. The hypocrites emphasize their belief in Allah and the Last Day with their words, when that is not the case in reality. Therefore, Allah stated that the hypocrites lie in their testimony of creed, when He said,
﴿وَاللَّهُ يَشْهَدُ إِنَّ الْمُنَـفِقِينَ لَكَـذِبُونَ﴾
(And Allah bears witness that the hypocrites are indeed liars.) (63:1), and,
﴿وَمَا هُم بِمُؤْمِنِينَ﴾
(while in fact they believe not)
Allah said,
﴿يُخَـدِعُونَ اللَّهَ وَالَّذِينَ ءَامَنُوا﴾
(They try to deceive Allah and those who believe). The hypocrites show belief outwardly while concealing disbelief. They think that by doing this, they will mislead Allah, or that the statements they utter will help them with Allah, and this is an indication of their total ignorance. They think that such behavior will deceive Allah, just as it might deceive some of the believers. Similarly, Allah said,
﴿يَوْمَ يَبْعَثُهُمُ اللَّهِ جَمِيعاً فَيَحْلِفُونَ لَهُ كَمَا يَحْلِفُونَ لَكُمْ وَيَحْسَبُونَ أَنَّهُمْ عَلَى شَىْءٍ أَلاَ إِنَّهُمْ هُمُ الْكَـذِبُونَ ﴾
(On the Day when Allah will resurrect them all together; then they will swear to Him as they swear to you. And they think that they have something (to stand upon). Verily, they are liars!) (58:18). aHence, Allah refuted their way by saying,
﴿وَمَا يَخْدَعُونَ إلاَّ أَنفُسَهُمْ وَمَا يَشْعُرُونَ﴾
(While they only deceive themselves, and perceive (it) not!) Allah stated that the hypocrites only deceive themselves by this behavior, although they are unaware of this fact. Allah also said,
﴿إِنَّ الْمُنَـفِقِينَ يُخَـدِعُونَ اللَّهَ وَهُوَ خَادِعُهُمْ﴾
(Verily, the hypocrites try to deceive Allah, but it is He Who deceives them) (4:142).
Also, Ibn Abi Hatim narrated that Ibn Jurayj commented on Allah's statement,
﴿إِنَّ الْمُنَـفِقِينَ يُخَـدِعُونَ اللَّهَ وَهُوَ خَادِعُهُمْ﴾
(Verily, the hypocrites seek to deceive Allah, but it is He Who deceives them), "The hypocrites pronounce, `There is no deity worthy of worship except Allah' seeking to ensure the sanctity of their blood and money, all the while concealing disbelief.''Sa`id said that Qatadah said,
﴿وَمِنَ النَّاسِ مَن يَقُولُ ءَامَنَّا بِاللَّهِ وَبِالْيَوْمِ الأْخِرِ وَمَا هُم بِمُؤْمِنِينَ - يُخَـدِعُونَ اللَّهَ وَالَّذِينَ ءَامَنُوا وَمَا يَخْدَعُونَ إلاَّ أَنفُسَهُمْ وَمَا يَشْعُرُونَ ﴾
(And of mankind, there are some who say: "We believe in Allah and the Last Day'' while in fact they believe not. They try to deceive Allah and those who believe, while they only deceive themselves, and perceive (it) not!) "This is the description of a hypocrite. He is devious, he says the truth with his tongue and defies it with his heart and deeds. He wakes up in a condition other than the one he goes to sleep in, and goes to sleep in a different condition than the one he wakes up in. He changes his mind just like a ship that moves about whenever a wind blows.''
﴿فِى قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ اللَّهُ مَرَضًا وَلَهُمْ عَذَابٌ أَلِيمٌ بِمَا كَانُواْ يَكْذِبُونَ ﴾
(10. In their hearts is a disease and Allah has increased their disease. A painful torment is theirs because they used to tell lies.)