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Allah, exalted is He, has explicitly affirmed that faith increases, saying:
โWhenever a surah is revealed, some of them say, โWhich of you has this increased in faith?โ As for those who believe, it has increased them in faith, and they rejoice.โ [al-Tawbah: 124]
This means increased certainty in faith and greater protection against doubt entering it. For the Messengerโs signs, his words, and the revelation sent down to him confirm one another. This brings greater certainty in faith, clearer insight in holding fast to it, and greater protection against the intrusion of doubt.
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Faith thus varies in degree with respect to certainty, strength, knowledge, and freedom from the intrusion of doubt. The stronger a personโs certainty concerning Allah and knowledge of Him, the less susceptible he is to doubts. Someone who believes in Allah without coming to know Him through reasoning from evidence is not like someone who has come to know Him in this way. Nor is someone who knows Him through one line of evidence like someone who knows Him through many. Likewise, someone who knows Him through proofs without witnessing the signs is not like someone who personally witnessed them in the presence of the Prophet, may Allah bless him and grant him peace, in the strength of certainty within his heart and his resistance to being shaken in his faith or led astray from it by Satan.
It is reported that the Messenger of Allah, may Allah bless him and grant him peace, said: โAmong my Companions are those whose faith is more firmly established in their hearts than the firmly anchored mountains.โ He also said of one of his CompanionsโI think it was Abลซ BakrโโWere the veil lifted for him, his certainty would not increase.โ
It is also related that Munkar and Nakฤซr came to Umar ibn al-Khattฤb and asked him, โWho is your Lord, and what is your religion?โ He replied, โAs for me, Allah is my Lord, Islam is my religion, and Muhammad, may Allah bless him and grant him peace, is my Prophet. But you twoโwho is your Lord, what is your religion, and who is your Prophet?โ They looked at one another and said, โIt is Umar,โ then departed from him. Then can