Firstly, the testimony “La ilahailla Allah” is also called “Tawhid al Uluhiyyah” as well as “Tawhid al `Ubudiyyah” because it entails belief in Oneness of Allah, and implies that He alone is worthy of worship.
Secondly, Iman is a deed of the tongue also. (So that if a hypocrite testifies, then, he is a believer so far as we are concerned; but not a believer in the sight of Allah).
Thirdly, Iman is belief in the heart, and testimony with the tongue. (That is, to know the truth with the heart, and testify with the tongue. This is the opinion of Imam Abu Haneefa and most of the juris consults).
Fourth, iman is an act of the heart, the tongue, and the limbs. (This is the opinion of Ahl al-Hadith, Imam Malik, Imam Shafe`ee, Imam Ahmed ibn Hanbal, and Awza`ee.
It is also reported of Imam Shafe`ee that he said that Iman is (three things: Au.): to believe in the heart, testify (both) with the tongue as well as with corresponding deeds. Therefore, he who does not meet with the first condition (but comes up with the second and the third, i.e. testimony with the tongue and the deed) is a hypocrite. He who does not come up with the second is an unbeliever. And he who does not come up with the third (that is, with belief in the heart and testimony with the tongue, but not with concomitant deeds: Au.) is a fasiq, who will enter the Fire (unless forgiven: Au.) but will not abide there forever.
As for whether Iman increases or not, some have said that if what we mean by Iman is the testimony of the heart, then surely it does not accept increase or decrease. However, some have said that it does not accept decrease, because if there is no testimony then there can be no Iman at all. But it does admit increase by means of evidences added through reasoning and contemplation. However, Sufyan b. `Uyayna has said that it can go on decreasing until nothing of it remains, since, according to Kirmani, what admits increase has to admit decrease.
Hence, and to sum up, ibn Battal has said that there is no difference of opinion among the majority of scholars that Iman is testimony with the tongue and deeds with the limbs, and that it does increase and decrease – increasing with good deeds and decreasing with evil deeds. This is the opinion of `Umar ibn al-Khattab, `Ali, Ibn Mas`ud, Muadh, Abu Darda, Ibn `Abbas, Ibn `Umar, `Ammar, Abu Hurayrah, Hudhayfa, Salman, `Abdullah b. Rawaha, Abu Umamah, Jundub b. `Abdullah, `Umayr b. Habib, and `Ayesha. And, from among the followers, Ka`b al-Ahbar, `Urwah, `Ata’, Ta’us, Mujahid, Sa`eed b. Jubayr and scores of others.