Faith increases by performing good deeds regularly and decreases by committing sins and being negligent with Allah The Almighty. Allah The Almighty Says (what means):
• {The believers are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely} [Quran 8:2]
• {Those to whom hypocrites said, "Indeed, the people have gathered against you, so fear them." But it [merely] increased them in faith, and they said, "Sufficient for us is Allah, and [He is] the best Disposer of affairs."} [Quran 3:173]
Handhalah ibn Ar-Rabee’, may Allah be pleased with him, met Abu Bakr As-Siddeeq, may Allah be pleased with him, and told him, “Hanthalah is a hypocrite!” Abu Bakr, may Allah be pleased with him, said, “Glory be to Allah! What are you saying?” Hanthalah, may Allah be pleased with him, said, “When we are with the Messenger of Allah he reminds us of Paradise and Hell as if we can see them with our eyes, but when we leave him we have intimate relations with our wives, we play with our children and we become concerned with our worldly matters.” Abu Bakr, may Allah be pleased with him, said, “By Allah, it is the same with me.”
So, both of them went to the Prophet and Hanthalah said to him, “O Messenger of Allah! Hanthalah is a hypocrite.” The Prophet said: “Why do you say this?” Hanthalah told the Prophet the same words that he had said to Abu Bakr. Then the Prophet said: “By Him in whose Hand is my life, if your state of mind remains the same as it is in my presence and you are always busy in remembering Allah, the angels will shake hands with you in your beds and in your paths but, Handhalah, there is a time for this and there is a time for that.” The Prophet said this thrice. [Muslim]
Abu'd-Dardaa’ used to say, “It is a sign of the slave’s religious understanding that he cares for his faith so that it would not decrease, and to know whether his faith is increasing or decreasing.”
There are means that increase faith, some of which are: knowledge, good deeds, reflection and so forth. Gaining more knowledge is a means of increasing faith as Ibn ‘Umar used to say, “We learned faith, but when we learned the Quran we increased in faith.” There is no doubt that having knowledge of the Names of Allah The Almighty and the meanings that each one of them comprises increases the Muslim’s faith. Knowing the Seerah (biography) of the Prophet along with his morals, dealings, Jihaad and acts of worship increases faith and strengthens it. Allah The Almighty praises people of knowledge in many places in the Quran. Allah The Almighty Says (what means): {Say, "Are those who know equal to those who do not know?"} [Quran 39:9]
It was narrated that Al-Haarith ibn Maalik Al-Ansaari was passing by and the Prophet asked him: “O Haarithah! How are you doing?” He replied, “I have become a true believer!” The Prophet told him: “Be wary of what you say, as everything has a truth, so what is the truth of your faith?” He said, “I abandoned the worldly life to the extent that I stay up at night (praying) and I remain thirsty during the day (because of fasting). I feel as if I can see the Throne of My Lord; the people of Paradise visiting each other and the people of Hell being tortured in it.” The Prophet said: “O Haarithah! You have come to know, so adhere to what you know!” He repeated this three times. [At-Tabaraani]
Just as knowledge is a means of increasing faith, deeds are also a means of either increasing or decreasing faith. Performing good deeds increases faith; committing sins weakens it. The Companions were keen on increasing their faith. ‘Umar used to say, “Let us make our faith increase.” And they would remember Allah The Almighty. ‘Abdullaah ibn Rawaahah used to hold the hand of his friends and say, “Get up and let us have a faith boosting hour (by sitting with people who remember Allah).”
It was narrated that Allah The Almighty Says: “I Am just as My slave thinks of Me and I am with him when he remembers Me. If he remembers me secretly, I remember him secretly and if he remembers Me in a gathering, I remember him in a gathering that is better.” [Al-Bukhari and Muslim]
Among the reasons that increase faith is constant reflection upon the creation of Allah The Almighty. If someone tells you about the cleverness of a certain person and you see a sample of his work, you would be more amazed. If you see even more examples, you would be even more amazed at his cleverness, and if you scrutinize his workmanship carefully, you would be even more convinced about his skill and truthfulness!
The might of Allah The Almighty is quite clear to everyone in this vast universe. The heavens are lifted without pillars and have stars and planets; while the earth is planted with seeds and produces the sweet and the sour. This makes the believer who has a pious heart unable to contain himself and he finds himself declaring the glory of Allah The Almighty and the beauty of His creation, saying, “Indeed, He is Allah, the Omnipotent.”
Reflecting upon the creation of Allah The Almighty is one of the characteristics of the believers. Allah The Almighty Says (what means): {And give thought to the creation of the heavens and the earth...} [Quran 3:191]