Praise be to Allah.
A person may become negligent and that may lead to weakening his faith. The remedy for that is to pray a great deal for forgiveness and to constantly remember Allah, and to read Quran and focus on and ponder its meanings, for doing that will rid the heart of its carelessness and wake it from its slumber. And it is very important to do a lot of righteous deeds.
There are several reasons for weakness of faith, including the following:
1 – Ignorance of the names and attributes of Allah leads to weakness of faith because if a person is lacking in knowledge of the names and attributes of Allah, his faith will be imcomplete.
2 – Neglecting to think of the signs of Allah in the universe and to ponder His commands and prohibitions. This causes a lack of faith or at the very least it makes faith stagnant and prevents it from growing.
3 – Committing sin, for sin has a far-reaching effect on the heart and on faith. Hence the Prophet (peace and blessings of Allah be upon him) said: “No adulterer is a believer at the moment when he is committing adultery.” (Narrated by al-Bukhaari, 2475; Muslim, 57).
4 – Failing to do acts of worship, for failing to do acts of worship diminishes faith. But if the act of worship is something that is obligatory and it is neglected with no excuse, then this is a shortcoming for which a person is to be blamed and will be punished. If it is an act of worship that is not obligatory or it is obligatory but a person does not do it because he has an excuse, then this is a shortcoming for which he will not be blamed or punished.
Majmoo’ Fataawa wa Rasaa’il Fadeelat al-Shaykh Muhammad ibn Saalih al-‘Uthaymeen, 1/52
We advise you to read Quran a great deal and ponder its verses, for in that there is healing for what is in your heart, if Allah wills. Allah says (interpretation of the meaning):
“And We send down of the Quran that which is a healing and a mercy to those who believe”
We also advise you to study the stories of the Prophets, for Allah narrated them in the Quran to make the heart of his Prophet (peace and blessings of Allah be upon him), and the believers with him, steadfast. Allah says (interpretation of the meaning):
“And all that We relate to you (O Muhammad) of the news of the Messengers is in order that We may make strong and firm your heart thereby. And in this (chapter of the Quran) has come to you the truth, as well as an admonition and a reminder for the believers” [Hood 11:120]
And we also advise you to read question no. 14041.
O Allah, make faith dear to us and make kufr (disbelief), evildoing and sin hateful to us, and make us of those who are rightly-guided.