The claim that faith is nothing more than knowledge is false because according to the Qur’an, knowledge can co-exist with disbelief. God says: Indeed those who turned their backs after the guidance had become clear to them… (47:25); Indeed those who are faithless and bar from the way of God and defy the Messenger after guidance has become clear to them… (47:32); They impugned them – though they were convinced in their hearts…’ (27:14); …and whom God has led astray knowingly… (45:23). These verses affirm the possibility of disbelief, apostasy and going astray despite having knowledge.
Therefore, just knowing something and being convinced of its truth is not sufficient to have faith. Rather, one must observe its practical implications. So, someone who comes to know tawhid (oneness of God) and then follows its practical implications, namely being God’s servant and worshiping Him alone, such a person can be called faithful. Conversely, someone who knows tawhid but does not put it into practice and does not act to demonstrate his servitude, he cannot be said to have faith. For the latter, we can use the example of Iblis. He had knowledge of the Lordship of the Creator and the Day of Resurrection (vide 15:36) but he failed in the practical implications – refusing to prostrate before Adam as commanded by God (vide 2:34).
The above also proves false the claim that faith is only deeds, because deeds can co-exist with hypocrisy (nifāq); a hypocrite (munāfiq) is someone who performs good deeds and perhaps even has knowledge of the truth, and yet he does not have faith.
Since knowledge and its practical implications can increase or decrease and strengthen or weaken, then faith – which is a combination of the two - can also be subject to increase or decrease and also strengthening or weakening. Therefore, that faith has different degrees and levels is a necessary truth which is beyond doubt. This is the consensus opinion of most scholars and supported by God’s words, that they might enhance their faith and other verses of the Qur’an. There are also numerous narrations that have been transmitted from the Imams of the Prophet’s Household which indicate that faith has numerous levels.
It is also a fact that faith can also be strengthened through persistence in good deeds or weakened as a result of sins. After all, the strength or weakness of the effect (that is, a person’s deeds) reveals the strength or weakness of its cause (that is, their faith). God says in the Qur’an: To Him ascends the good word, and righteous conduct elevates it (35:10) and Then the fate of those who committed misdeeds was that they denied the signs of God and they used to deride them (30:10).
Reference: Al-Mizan Ar. Vol. 18, under Ch. 48.